Add filter for backend to rest index

This adds a "backend" type filter to the index REST route which is a pre-requisite for #12620

For example when calling `index.php/settings/users/users?offset=0&limit=10&gid=&pattern=&backend=OC_User_Database` only users within the backend `OC_User_Database` would be shown. (requires sending a CSRF token as well)
Suspicious: `$this->groupManage->displayNamesInGroup()` returns an array of $uid => $displayName, but `getUsersForUID()` iterates with `foreach ($userIds as $userId)` which would iterate over the `$displayName`.
Please double check.