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)
Why not \OCP\UserInterface (and change it in every backend;) I know you will love it )?