Backup Datastore using the Admin REST API

Add server end points to backup Datastore using managed-export mechanism.
A cron job is defined in Alpha to run daily exports using this implementation.

Existing backup is left running. The new backups are saved to a new set of
locations:
- GCS bucket: gs://PROJECT-ID-datastore-backups
- Big Query data set: datastore_backups
- Big Query latest back up view name: latest_datastore_backup
Also, the names of Bigquery tables now use the export timestamp
assigned by Datastore. E.g., 2018_12_05T23_56_18_50532_ContactResource,

After the new import mechanism is implemented and the back-restore flow is
tested, we will stop the existing backup runs and deploy the new
implementation to all environments.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=224932957

java/google/registry/config/CredentialModule.java

@@ -40,7 +40,21 @@ import javax.inject.Singleton;
 @Module
 public abstract class CredentialModule {
 
-  /** Provides the default {@link GoogleCredential} from the Google Cloud runtime. */
+  /**
+   * Provides the default {@link GoogleCredential} from the Google Cloud runtime.
+   *
+   * <p>The credential returned depends on the runtime environment:
+   *
+   * <ul>
+   *   <li>On AppEngine, returns the service account credential for
+   *       PROJECT_ID@appspot.gserviceaccount.com
+   *   <li>On Compute Engine, returns the service account credential for
+   *       PROJECT_NUMBER-compute@developer.gserviceaccount.com
+   *   <li>On end user host, this returns the credential downloaded by gcloud. Please refer to <a
+   *       href="https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login">Cloud
+   *       SDK documentation</a> for details.
+   * </ul>
+   */
   @DefaultCredential
   @Provides
   @Singleton