ADC Authentication support for Integration tests in gcsio layer (GoogleCloudDataproc#1361)

* ADC Authentication support

* Removed unnecessary  comments

* Refactor: Centralize GCS_TEST_APPLICATION_DEFAULT_ENABLE config access

Previously, GCS_TEST_APPLICATION_DEFAULT_ENABLE was retrieved directly
within the code consuming it. This commit moves the retrieval logic to
`TestConfiguration.java`.

* Added null check for environment variable and removed whitespaces by mvn spotless:apply

* Removed empty lines in the code

* Resolved spotless:apply iisues

* Modified nested if else block in GetCredentials and replaced it with ternary operator in GoogleCloudStorageTestHelper.java for better code readability.

---------

Co-authored-by: Animesh Gupta <animgupt@google.com>

Author: yash-jainn

gcsio/src/test/java/com/google/cloud/hadoop/gcsio/integration/GoogleCloudStorageTestHelper.java

@@ -29,6 +29,7 @@
 import com.google.api.services.storage.model.StorageObject;
 import com.google.auth.Credentials;
 import com.google.auth.oauth2.ComputeEngineCredentials;
+import com.google.auth.oauth2.GoogleCredentials;
 import com.google.auth.oauth2.ServiceAccountCredentials;
 import com.google.cloud.hadoop.gcsio.GoogleCloudStorage;
 import com.google.cloud.hadoop.gcsio.GoogleCloudStorageClientImpl;
@@ -101,7 +102,11 @@ public static Credentials getCredentials() throws IOException {
     String serviceAccountJsonKeyFile =
         TestConfiguration.getInstance().getServiceAccountJsonKeyFile();
     if (serviceAccountJsonKeyFile == null) {
-      return ComputeEngineCredentials.create().createScoped(StorageScopes.CLOUD_PLATFORM);
+      Boolean isApplicationDefaultModeEnabled =
+          TestConfiguration.getInstance().isApplicationDefaultModeEnabled();
+      return isApplicationDefaultModeEnabled
+          ? GoogleCredentials.getApplicationDefault()
+          : ComputeEngineCredentials.create().createScoped(StorageScopes.CLOUD_PLATFORM);
     }
     try (FileInputStream fis = new FileInputStream(serviceAccountJsonKeyFile)) {
       return ServiceAccountCredentials.fromStream(fis).createScoped(StorageScopes.CLOUD_PLATFORM);