Create group-indexed extension point

Change-Id: I79fb7404e10e619d87b4983346c90ca79c83ddff

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/GroupIndexedListener.java

@@ -0,0 +1,28 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+/** Notified whenever a group is indexed */
+@ExtensionPoint
+public interface GroupIndexedListener {
+  /**
+   * Invoked when a group is indexed
+   *
+   * @param uuid of the group
+   */
+  void onGroupIndexed(String uuid);
+}