Do not compute extended groups when adding a user to a group

The call to "POST /user/

/group/

" to add a user to a group may be very slow if the user belongs to many groups which belong to many parent groups

In this case, the computation of the extended groups will take a long time where the extended groups are not useful or used when running this command.

There should be at least a query parameter not to fetch the references. And maybe the default behavior could be not to use the references on the groups.

The Java client needs to be updated to take advantage of this new parameter.