util: introduce wl_object_get_{id,class}()

See https://lists.freedesktop.org/archives/wayland-devel/2024-April/043570.html for motivation; my compositor has a similar requirement so I'd like to be able to get the required information even with WL_HIDE_DEPRECATED.

Added the new functions to util because that's where struct wl_object is declared, although normally clients would never access wl_objects AFAIU so it could belong to server as well.