[Tests] Extract the testing logic for Google Cloud Storage (#28576)

This pull request extracts in a dedicated class the request/response 
logic that "emulates" a Google Cloud Storage service in our 
repository-gcs tests.

The idea behind this is to make the logic more reusable. The class 
MockHttpTransport has been renamed to MockStorage which now 
only takes care of instantiating a Storage client and does the low-level 
request/response plumbing needed by this client.

The "Google Cloud Storage" logic has been extracted from 
MockHttpTransport and put in a new GoogleCloudStorageTestServer 
that is now independent from the google client testing framework.

plugins/repository-gcs/src/test/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageBlobStoreContainerTests.java

@@ -23,13 +23,13 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.repositories.ESBlobStoreContainerTestCase;
 
-import java.io.IOException;
 import java.util.Locale;
 
 public class GoogleCloudStorageBlobStoreContainerTests extends ESBlobStoreContainerTestCase {
+
     @Override
-    protected BlobStore newBlobStore() throws IOException {
+    protected BlobStore newBlobStore() {
         String bucket = randomAlphaOfLength(randomIntBetween(1, 10)).toLowerCase(Locale.ROOT);
-        return new GoogleCloudStorageBlobStore(Settings.EMPTY, bucket, MockHttpTransport.newStorage(bucket, getTestName()));
+        return new GoogleCloudStorageBlobStore(Settings.EMPTY, bucket, MockStorage.newStorageClient(bucket, getTestName()));
     }
 }

plugins/repository-gcs/src/test/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageBlobStoreRepositoryTests.java

@@ -65,7 +65,7 @@ protected void createTestRepository(String name) {
 
     @BeforeClass
     public static void setUpStorage() {
-        storage.set(MockHttpTransport.newStorage(BUCKET, GoogleCloudStorageBlobStoreRepositoryTests.class.getName()));
+        storage.set(MockStorage.newStorageClient(BUCKET, GoogleCloudStorageBlobStoreRepositoryTests.class.getName()));
     }
 
     public static class MockGoogleCloudStoragePlugin extends GoogleCloudStoragePlugin {

plugins/repository-gcs/src/test/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageBlobStoreTests.java

@@ -23,14 +23,13 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.repositories.ESBlobStoreTestCase;
 
-import java.io.IOException;
 import java.util.Locale;
 
 public class GoogleCloudStorageBlobStoreTests extends ESBlobStoreTestCase {
 
     @Override
-    protected BlobStore newBlobStore() throws IOException {
+    protected BlobStore newBlobStore() {
         String bucket = randomAlphaOfLength(randomIntBetween(1, 10)).toLowerCase(Locale.ROOT);
-        return new GoogleCloudStorageBlobStore(Settings.EMPTY, bucket, MockHttpTransport.newStorage(bucket, getTestName()));
+        return new GoogleCloudStorageBlobStore(Settings.EMPTY, bucket, MockStorage.newStorageClient(bucket, getTestName()));
     }
 }