Add REST endpoint to reindex a single group

This may become handy to fix single groups that are stale in the
index.

Change-Id: Iab3711f343fe1da9bba4979792edc83c76d75a09
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/groups/GroupApi.java

@@ -143,6 +143,15 @@ public interface GroupApi {
    */
   List<? extends GroupAuditEventInfo> auditLog() throws RestApiException;
 
+  /**
+   * Reindexes the group.
+   *
+   * Only supported for internal groups.
+   *
+   * @throws RestApiException
+   */
+  void index() throws RestApiException;
+
   /**
    * A default implementation which allows source compatibility
    * when adding new methods to the interface.
@@ -239,5 +248,10 @@ public interface GroupApi {
         throws RestApiException {
       throw new NotImplementedException();
     }
+
+    @Override
+    public void index() {
+      throw new NotImplementedException();
+    }
   }
 }