Store WorkspaceID in the API Client (#647)

## What changes are proposed in this pull request?
Store WorkspaceID in the API Client. 

This will be used to inject headers in Unified Mode.

## How is this tested?
Added unit tests

NO_CHANGELOG=true

Author: hectorcast-db

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

@@ -42,6 +42,7 @@ public static class Builder {
     private Integer debugTruncateBytes;
     private HttpClient httpClient;
     private String accountId;
+    private String workspaceId;
     private RetryStrategyPicker retryStrategyPicker;
     private boolean isDebugHeaders;
 
@@ -52,6 +53,7 @@ public Builder withDatabricksConfig(DatabricksConfig config) {
       this.httpClient = config.getHttpClient();
       this.debugTruncateBytes = config.getDebugTruncateBytes();
       this.accountId = config.getAccountId();
+      this.workspaceId = config.getWorkspaceId();
       this.isDebugHeaders = config.isDebugHeaders();
 
       if (config.getDisableRetries()) {
@@ -114,6 +116,7 @@ public ApiClient build() {
   private final Function<Void, String> getHostFunc;
   private final Function<Void, String> getAuthTypeFunc;
   private final String accountId;
+  private final String workspaceId;
   private final boolean isDebugHeaders;
   private static final String RETRY_AFTER_HEADER = "retry-after";
 
@@ -125,6 +128,10 @@ public String configuredAccountID() {
     return accountId;
   }
 
+  public String workspaceId() {
+    return workspaceId;
+  }
+
   public ApiClient(DatabricksConfig config) {
     this(config, new SystemTimer());
   }
@@ -143,6 +150,7 @@ private ApiClient(Builder builder) {
     this.getAuthTypeFunc = builder.getAuthTypeFunc != null ? builder.getAuthTypeFunc : v -> "";
     this.httpClient = builder.httpClient;
     this.accountId = builder.accountId;
+    this.workspaceId = builder.workspaceId;
     this.retryStrategyPicker =
         builder.retryStrategyPicker != null
             ? builder.retryStrategyPicker

databricks-sdk-java/src/test/java/com/databricks/sdk/core/ApiClientTest.java

@@ -483,4 +483,30 @@ void privateLinkRedirectBecomesPrivateLinkValidationError() throws MalformedURLE
                     new Request("GET", req.getUri().getPath()), MyEndpointResponse.class));
     assertTrue(e.getMessage().contains("AWS PrivateLink"));
   }
+
+  @Test
+  void testDefaultWorkspaceIdReturnsNullWhenNotSet() {
+    Request req = getBasicRequest();
+    DatabricksConfig config =
+        new DatabricksConfig()
+            .setHost("http://my.host")
+            .setCredentialsProvider(new DummyCredentialsProvider());
+    ApiClient client =
+        getApiClient(config, req, Collections.singletonList(getSuccessResponse(req)));
+    assertNull(client.workspaceId());
+  }
+
+  @Test
+  void testDefaultWorkspaceIdReturnsValueFromConfig() {
+    String expectedWorkspaceId = "test-workspace-123";
+    Request req = getBasicRequest();
+    DatabricksConfig config =
+        new DatabricksConfig()
+            .setHost("http://my.host")
+            .setWorkspaceId(expectedWorkspaceId)
+            .setCredentialsProvider(new DummyCredentialsProvider());
+    ApiClient client =
+        getApiClient(config, req, Collections.singletonList(getSuccessResponse(req)));
+    assertEquals(expectedWorkspaceId, client.workspaceId());
+  }
 }