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)
Calling `getUsersForUID` with `array_keys()` would have worked too. Not sure about the performance though.