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/RegistryConfig.java

@@ -1384,6 +1384,15 @@ public final class RegistryConfig {
     return getProjectId() + "-snapshots";
   }
 
+  /**
+   * Returns the Google Cloud Storage bucket for storing Datastore backups.
+   *
+   * @see google.registry.export.BackupDatastoreAction
+   */
+  public static String getDatastoreBackupsBucket() {
+    return "gs://" + getProjectId() + "-datastore-backups";
+  }
+
   /**
    * Number of sharded commit log buckets.
    *