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/export/datastore/Operation.java

@@ -68,6 +68,10 @@ public class Operation extends GenericJson {
     return getState().equals(STATE_PROCESSING);
   }
 
+  /**
+   * Returns the elapsed time since starting if this operation is still running, or the total
+   * running time if this operation has completed.
+   */
   public Duration getRunningTime(Clock clock) {
     return new Duration(
         getStartTime(), getMetadata().getCommonMetadata().getEndTime().orElse(clock.nowUtc()));