CreateGroup: add members to GroupInput

Currently in CreateGroup, we use either the ownerId owner group from
input, or the current calling user.

It would be useful to allow customized initial members, for situations
like create a group for a robot account.

Change-Id: I3685381fee6745e7ee3860f0c3757572bfec60aa

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

@@ -14,9 +14,12 @@
 
 package com.google.gerrit.extensions.api.groups;
 
+import java.util.List;
+
 public class GroupInput {
   public String name;
   public String description;
   public Boolean visibleToAll;
   public String ownerId;
+  public List<String> members;
 }