Support datastore restore in Nomulus tool

Two commands are being added:
- ImportDatastoreCommand starts an async import operation.
  User may choose to wait until import completes or quit
  immediately.
- GetOperationStatusCommand checks the status of an operation.
  It may be used to check the status of an operation started by
  ImportDatastoreCommand.

Both commands communicate with Datastore admin api directly, without
going through the Registry server.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228400059

java/google/registry/export/datastore/DatastoreAdmin.java

@@ -25,6 +25,7 @@ import com.google.api.client.json.GenericJson;
 import com.google.api.client.json.JsonFactory;
 import com.google.api.client.util.Key;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
 import java.util.Collection;
 import java.util.Optional;
 
@@ -73,12 +74,32 @@ public class DatastoreAdmin extends AbstractGoogleJsonClient {
    * </ul>
    *
    * @param outputUrlPrefix the full resource URL of the external storage location
-   * @param kinds the datastore 'kinds' to be exported
+   * @param kinds the datastore 'kinds' to be exported. If empty, all kinds will be exported
    */
   public Export export(String outputUrlPrefix, Collection<String> kinds) {
     return new Export(new ExportRequest(outputUrlPrefix, kinds));
   }
 
+  /**
+   * Imports the entire backup specified by {@code backupUrl} back to Cloud Datastore.
+   *
+   * <p>A successful backup restores deleted entities and reverts updates to existing entities since
+   * the backup time. However, it does not affect newly added entities.
+   */
+  public Import importBackup(String backupUrl) {
+    return new Import(new ImportRequest(backupUrl, ImmutableList.of()));
+  }
+
+  /**
+   * Imports the backup specified by {@code backupUrl} back to Cloud Datastore. Only entities whose
+   * types are included in {@code kinds} are imported.
+   *
+   * @see #importBackup(String)
+   */
+  public Import importBackup(String backupUrl, Collection<String> kinds) {
+    return new Import(new ImportRequest(backupUrl, kinds));
+  }
+
   /**
    * Returns a {@link Get} request that retrieves the details of an export or import {@link
    * Operation}.
@@ -158,6 +179,20 @@ public class DatastoreAdmin extends AbstractGoogleJsonClient {
     }
   }
 
+  /** A request to restore an backup to a Cloud Datastore database. */
+  public class Import extends DatastoreAdminRequest<Operation> {
+
+    Import(ImportRequest importRequest) {
+      super(
+          DatastoreAdmin.this,
+          "POST",
+          "projects/{projectId}:import",
+          importRequest,
+          Operation.class);
+      set("projectId", projectId);
+    }
+  }
+
   /** A request to retrieve details of an export or import operation. */
   public class Get extends DatastoreAdminRequest<Operation> {
 
@@ -216,8 +251,27 @@ public class DatastoreAdmin extends AbstractGoogleJsonClient {
 
     ExportRequest(String outputUrlPrefix, Collection<String> kinds) {
       checkNotNull(outputUrlPrefix, "outputUrlPrefix");
+      checkArgument(!kinds.isEmpty(), "kinds must not be empty");
       this.outputUrlPrefix = outputUrlPrefix;
       this.entityFilter = new EntityFilter(kinds);
     }
   }
+
+  /**
+   * Model object that describes the JSON content in an export request.
+   *
+   * <p>Please note that some properties defined in the API are excluded, e.g., {@code databaseId}
+   * (not supported by Cloud Datastore) and labels (not used by Domain Registry).
+   */
+  @SuppressWarnings("unused")
+  static class ImportRequest extends GenericJson {
+    @Key private final String inputUrl;
+    @Key private final EntityFilter entityFilter;
+
+    ImportRequest(String inputUrl, Collection<String> kinds) {
+      checkNotNull(inputUrl, "outputUrlPrefix");
+      this.inputUrl = inputUrl;
+      this.entityFilter = new EntityFilter(kinds);
+    }
+  }
 }