Add filter for 'backend' to user REST route

This adds a "backend" type filter to the index REST route which is a pre-requisite for https://github.com/owncloud/core/issues/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)

Depends upon https://github.com/owncloud/core/pull/12711