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/module/backend/BackendComponent.java

@@ -22,6 +22,7 @@ import google.registry.config.CredentialModule;
 import google.registry.config.RegistryConfig.ConfigModule;
 import google.registry.dns.writer.VoidDnsWriterModule;
 import google.registry.export.DriveModule;
+import google.registry.export.datastore.DatastoreAdminModule;
 import google.registry.export.sheet.SheetsServiceModule;
 import google.registry.gcs.GcsServiceModule;
 import google.registry.groups.DirectoryModule;
@@ -56,6 +57,7 @@ import javax.inject.Singleton;
       BigqueryModule.class,
       ConfigModule.class,
       CredentialModule.class,
+      DatastoreAdminModule.class,
       DatastoreServiceModule.class,
       DirectoryModule.class,
       DummyKeyringModule.class,