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/Operation.java

@@ -15,10 +15,10 @@
 package google.registry.export.datastore;
 
 import static com.google.common.base.Preconditions.checkState;
+import static com.google.common.base.Strings.isNullOrEmpty;
 
 import com.google.api.client.json.GenericJson;
 import com.google.api.client.util.Key;
-import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import google.registry.export.datastore.DatastoreAdmin.Get;
@@ -52,6 +52,14 @@ public class Operation extends GenericJson {
     return name;
   }
 
+  public boolean isExport() {
+    return !isNullOrEmpty(getExportFolderUrl());
+  }
+
+  public boolean isImport() {
+    return !isNullOrEmpty(getMetadata().getInputUrl());
+  }
+
   public boolean isDone() {
     return done;
   }
@@ -88,7 +96,9 @@ public class Operation extends GenericJson {
   /**
    * Returns the URL to the GCS folder that holds the exported data. This folder is created by
    * Datastore and is under the {@code outputUrlPrefix} set to {@linkplain
-   * DatastoreAdmin#export(String, List) the export request}.
+   * DatastoreAdmin#export(String, java.util.Collection) the export request}.
+   *
+   * @throws IllegalStateException if this is not an export operation
    */
   public String getExportFolderUrl() {
     return getMetadata().getOutputUrlPrefix();
@@ -98,6 +108,8 @@ public class Operation extends GenericJson {
    * Returns the last segment of the {@linkplain #getExportFolderUrl() export folder URL} which can
    * be used as unique identifier of this export operation. This is a better ID than the {@linkplain
    * #getName() operation name}, which is opaque.
+   *
+   * @throws IllegalStateException if this is not an export operation
    */
   public String getExportId() {
     String exportFolderUrl = getExportFolderUrl();
@@ -138,12 +150,12 @@ public class Operation extends GenericJson {
     public CommonMetadata() {}
 
     String getOperationType() {
-      checkState(!Strings.isNullOrEmpty(operationType), "operationType may not be null or empty");
+      checkState(!isNullOrEmpty(operationType), "operationType may not be null or empty");
       return operationType;
     }
 
     String getState() {
-      checkState(!Strings.isNullOrEmpty(state), "state may not be null or empty");
+      checkState(!isNullOrEmpty(state), "state may not be null or empty");
       return state;
     }
 
@@ -165,6 +177,7 @@ public class Operation extends GenericJson {
     @Key private Progress progressEntities;
     @Key private Progress progressBytes;
     @Key private EntityFilter entityFilter;
+    @Key private String inputUrl;
     @Key private String outputUrlPrefix;
 
     public Metadata() {}
@@ -186,9 +199,22 @@ public class Operation extends GenericJson {
       return entityFilter;
     }
 
+    public String getInputUrl() {
+      return checkUrls().inputUrl;
+    }
+
     public String getOutputUrlPrefix() {
-      checkState(!Strings.isNullOrEmpty(outputUrlPrefix), "outputUrlPrefix");
-      return outputUrlPrefix;
+      return checkUrls().outputUrlPrefix;
+    }
+
+    Metadata checkUrls() {
+      checkState(
+          isNullOrEmpty(inputUrl) || isNullOrEmpty(outputUrlPrefix),
+          "inputUrl and outputUrlPrefix must not be both present");
+      checkState(
+          !isNullOrEmpty(inputUrl) || !isNullOrEmpty(outputUrlPrefix),
+          "inputUrl and outputUrlPrefix must not be both missing");
+      return this;
     }
   }