Fix method visibility in JsonApiExtra

[yenshirak]

If you load the Jsonapi extra and include Pagy::Backend in a controller then pagy_url_for and pagy_set_query_params will be added as public methods. Only the controller methods that are mapped to routes should be public.

[ddnexus]

Thank you!
@yenshirak FYI, the upcoming v10 will not have any scattered around methods like this anymore.