Propagate Request Headers through API Client (#135)

## Changes
Based on changes in
databricks/databricks-sdk-go#572.

This PR adds initial support for headers in ApiClient.java. The `*Impl`
classes, when making a request, will construct a map of headers to be
included in the request. These are passed through a new parameter in
each of GET/PUT/POST/PATCH/DELETE.

In the future, to support dynamic headers, we can modify our codegen
template to add those fields to the headers map before calling the
ApiClient.

Alternatives considered:
* **Adding headers in request objects directly**: this would not require
any change in the ApiClient interface. Instead, we would introduce a new
annotation for headers, much like what we do for query parameters, and
reflectively scan the fields of each request object. We could do this,
but for headers that have a fixed value (like `Content-Type` and
`Accept`), these fields would essentially be private final fields that
we could iterate through. Users can never interact with these fields, so
it seems a bit unusual to have them in the request structure, a public
interface.

## Tests
<!-- How is this tested? -->

Author: mgyucht

.codegen/impl.java.tmpl

@@ -4,6 +4,7 @@ package com.databricks.sdk.service.{{.Package.Name}};
 import java.io.IOException;
 import java.util.Collection;
 import java.util.Map;
+import java.util.HashMap;
 
 import com.databricks.sdk.core.ApiClient;
 import com.databricks.sdk.core.DatabricksException;
@@ -26,18 +27,26 @@ class {{.PascalName}}Impl implements {{.PascalName}}Service {
     String path = {{if .PathParts -}}
       String.format("{{range  .PathParts}}{{.Prefix}}{{if or .Field .IsAccountId}}%s{{end}}{{ end }}"{{ range .PathParts }}{{if .Field}}, request.get{{.Field.PascalName}}(){{ else if .IsAccountId }}, apiClient.configuredAccountID(){{end}}{{ end }})
     {{- else}}"{{.Path}}"{{end}};
+    {{ template "headers" . }}
     {{if .Response -}}
-      {{- if .Response.ArrayValue -}} return apiClient.getCollection(path, null, {{template "type" .Response.ArrayValue}}.class);
-      {{- else if .Response.MapValue -}} return apiClient.getStringMap(path, request);
+      {{- if .Response.ArrayValue -}} return apiClient.getCollection(path, null, {{template "type" .Response.ArrayValue}}.class, headers);
+      {{- else if .Response.MapValue -}} return apiClient.getStringMap(path, request, headers);
       {{- else -}} return apiClient.{{.Verb}}(path{{if .Request}}, request{{end}}{{if .Response -}}, {{if .Response.ArrayValue }}Collection
                         {{- else if .Response.MapValue }}Map
                         {{- else}}{{template "type" .Response}}
-                        {{- end -}}{{else}}, Void{{end}}.class);{{end}}
+                        {{- end -}}{{else}}, Void{{end}}.class, headers);{{end}}
     {{- else -}}apiClient.{{.Verb}}(path{{if .Request}}, request{{end}}{{if .Response -}}, {{if .Response.ArrayValue }}Collection
       {{- else if .Response.MapValue }}Map
       {{- else}}{{template "type" .Response}}
-      {{- end -}}{{else}}, Void{{end}}.class);
+      {{- end -}}{{else}}, Void{{end}}.class, headers);
     {{end}}
   }
   {{end}}
-}
+}
+
+{{ define "headers" -}}
+    Map<String, String> headers = new HashMap<>();
+    {{- range $key, $value := .FixedRequestHeaders }}
+    headers.put("{{$key}}", "{{$value}}");
+    {{- end -}}
+{{- end }}

databricks-sdk-java/src/main/java/com/databricks/sdk/core/ApiClient.java

@@ -13,9 +13,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import java.io.IOException;
-import java.util.Collection;
-import java.util.Map;
-import java.util.Random;
+import java.util.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -85,81 +83,105 @@ private ObjectMapper makeObjectMapper() {
     return mapper;
   }
 
-  private <I> Request withQuery(Request in, I entity) {
+  private static <I> void setQuery(Request in, I entity) {
     if (entity == null) {
-      return in;
+      return;
     }
     for (GrpcTranscodingQueryParamsSerializer.HeaderEntry e :
         GrpcTranscodingQueryParamsSerializer.serialize(entity)) {
       in.withQueryParam(e.getKey(), e.getValue());
     }
-    return in;
   }
 
-  public <I, O> Collection<O> getCollection(String path, I in, Class<O> element) {
+  private static <I> void setHeaders(Request in, Map<String, String> headers) {
+    if (headers == null) {
+      return;
+    }
+    for (Map.Entry<String, String> e : headers.entrySet()) {
+      in.withHeader(e.getKey(), e.getValue());
+    }
+  }
+
+  public <I, O> Collection<O> getCollection(
+      String path, I in, Class<O> element, Map<String, String> headers) {
     return withJavaType(
-        path, in, mapper.getTypeFactory().constructCollectionType(Collection.class, element));
+        path,
+        in,
+        mapper.getTypeFactory().constructCollectionType(Collection.class, element),
+        headers);
   }
 
-  public <I> Map<String, String> getStringMap(String path, I in) {
+  public <I> Map<String, String> getStringMap(String path, I in, Map<String, String> headers) {
     return withJavaType(
-        path, in, mapper.getTypeFactory().constructMapType(Map.class, String.class, String.class));
+        path,
+        in,
+        mapper.getTypeFactory().constructMapType(Map.class, String.class, String.class),
+        headers);
   }
 
-  protected <I, O> O withJavaType(String path, I in, JavaType javaType) {
+  protected <I, O> O withJavaType(
+      String path, I in, JavaType javaType, Map<String, String> headers) {
     try {
-      Request request = withQuery(new Request("GET", path), in);
+      Request request = prepareRequest("GET", path, in, headers);
       Response response = getResponse(request);
       return deserialize(response.getBody(), javaType);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
-  public <O> O GET(String path, Class<O> target) {
-    return GET(path, null, target);
+  public <O> O GET(String path, Class<O> target, Map<String, String> headers) {
+    return GET(path, null, target, headers);
   }
 
-  public <I, O> O GET(String path, I in, Class<O> target) {
+  public <I, O> O GET(String path, I in, Class<O> target, Map<String, String> headers) {
     try {
-      return execute(withQuery(new Request("GET", path), in), target);
+      return execute(prepareRequest("GET", path, in, headers), target);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
-  public <I, O> O POST(String path, I in, Class<O> target) {
+  public <I, O> O POST(String path, I in, Class<O> target, Map<String, String> headers) {
     try {
-      return execute(new Request("POST", path, serialize(in)), target);
+      return execute(prepareRequest("POST", path, in, headers), target);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
-  public <I, O> O PUT(String path, I in, Class<O> target) {
+  public <I, O> O PUT(String path, I in, Class<O> target, Map<String, String> headers) {
     try {
-      return execute(new Request("PUT", path, serialize(in)), target);
+      return execute(prepareRequest("PUT", path, in, headers), target);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
-  public <I, O> O PATCH(String path, I in, Class<O> target) {
+  public <I, O> O PATCH(String path, I in, Class<O> target, Map<String, String> headers) {
     try {
-      return execute(new Request("PATCH", path, serialize(in)), target);
+      return execute(prepareRequest("PATCH", path, in, headers), target);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
-  public <I, O> O DELETE(String path, I in, Class<O> target) {
+  public <I, O> O DELETE(String path, I in, Class<O> target, Map<String, String> headers) {
     try {
-      return execute(withQuery(new Request("DELETE", path), in), target);
+      return execute(prepareRequest("DELETE", path, in, headers), target);
     } catch (IOException e) {
       throw new DatabricksException("IO error: " + e.getMessage(), e);
     }
   }
 
+  private <I> Request prepareRequest(
+      String method, String path, I in, Map<String, String> headers) {
+    Request req = new Request(method, path);
+    setQuery(req, in);
+    setHeaders(req, headers);
+    return req;
+  }
+
   /**
    * Executes HTTP request with retries and converts it to proper POJO
    *
@@ -177,7 +199,6 @@ private <T> T execute(Request in, Class<T> target) throws IOException {
 
   private Response getResponse(Request in) {
     in.withUrl(config.getHost() + in.getUrl());
-    in.withHeader("Accept", "application/json");
     return executeInner(in);
   }