Add osk stub to fullfil session dependencies

0. This helps to fullfil gnome-session dependencies on desktops where we don't have squeekboard (yet) and where it should not unfold (squeekboard#132 (closed)) allowing an upload to pureos proper so people can use it instead of / alongside gnome-shell

1. This can become the multiplexer instead of /usr/bin/osk-wayland in the future making proper keyboard selection part of the shell package.

2. This will also ease creating a test stub to test phosh<->osk protocol interaction in the future.

3. this allows for distro packaging where there's no librem-5-base (and in many cases never will be) and no squeekboard yet. Together with 1. we can add squeekboard later on without any bumps.