This API creates a "client" then later sets up a wl_client and returns a
file descriptor some Wayland client can connect to. It's meant to be
used as a method other than WAYLAND_SOCKET and process launching, e.g.
passing a file descriptor via a D-Bus API.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2810>
