Add gnome-ssh-askpass4 for GNOME 40+

Add gnome-ssh-askpass4 for GNOME 40+

GTK 3 has been in maintenance mode for a while now, and it is on the road
to being abandoned. As a result, the dialogue looks out of place on modern
systems.

We could port it to GTK 4 but without the program being registered as an
application (i.e. having a .desktop file), GNOME Shell would ask for
permission to grab input every time.

Let’s instead use the GNOME Shell’s native prompt through the unstable
Gcr API.