[WIP] add optional countUsersInGroup method to group backends

Required for https://github.com/owncloud/core/pull/7151

Currently the only method to count users in a group is to count the entries in the array returned by usersInGroup(). This has the disadvantage to  have high overhead and is especially dead slow with LDAP. Just counting users saves a lot of otherwise unnecessary additional treatment with users.

WIP – do not merge yet.