Change DatabricksOAuthTokenSource to be refreshable (#446)

## What changes are proposed in this pull request?

## Make DatabricksOAuthTokenSource refreshable

This change extends DatabricksOAuthTokenSource from
RefreshableTokenSource to enable automatic token refresh functionality
while maintaining the existing OAuth token exchange flow

## How is this tested?
Unit testing

NO_CHANGELOG=true

Author: emmyzhou-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/DatabricksOAuthTokenSource.java

@@ -17,7 +17,7 @@
  * Implementation of TokenSource that handles OAuth token exchange for Databricks authentication.
  * This class manages the OAuth token exchange flow using ID tokens to obtain access tokens.
  */
-public class DatabricksOAuthTokenSource implements TokenSource {
+public class DatabricksOAuthTokenSource extends RefreshableTokenSource {
   private static final Logger LOG = LoggerFactory.getLogger(DatabricksOAuthTokenSource.class);
 
   /** OAuth client ID used for token exchange. */
@@ -154,7 +154,7 @@ private static void validate(Object value, String fieldName) {
    *     parameters are missing.
    */
   @Override
-  public Token getToken() {
+  public Token refresh() {
     // Validate all required parameters
     validate(clientId, "ClientID");
     validate(host, "Host");

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/TokenSourceCredentialsProvider.java

@@ -41,12 +41,14 @@ public TokenSourceCredentialsProvider(TokenSource tokenSource, String authType)
   @Override
   public HeaderFactory configure(DatabricksConfig config) {
     try {
-      // Validate that we can get a token before returning the HeaderFactory
-      String accessToken = tokenSource.getToken().getAccessToken();
+      // Validate that we can get a token before returning a HeaderFactory
+      tokenSource.getToken().getAccessToken();
 
       return () -> {
         Map<String, String> headers = new HashMap<>();
-        headers.put("Authorization", "Bearer " + accessToken);
+        // Some TokenSource implementations cache tokens internally, so an additional getToken()
+        // call is not costly
+        headers.put("Authorization", "Bearer " + tokenSource.getToken().getAccessToken());
         return headers;
       };
     } catch (Exception e) {

databricks-sdk-java/src/test/java/com/databricks/sdk/core/oauth/TokenSourceCredentialsProviderTest.java

@@ -40,7 +40,8 @@ void testTokenScenarios(
       Map<String, String> headers = headerFactory.headers();
       assertEquals(expectedAuthHeader, headers.get("Authorization"));
     }
-    verify(mockTokenSource).getToken();
+
+    verify(mockTokenSource, atLeastOnce()).getToken();
     assertEquals(TEST_AUTH_TYPE, provider.authType());
   }