nixos/cage: init

Add a cage module to nixos. This can be used to make kiosk-style
systems that boot directly to a single application. The user (demo by
default) is automatically logged in by this service and the
program (xterm by default) is automatically started.

This is useful for some embedded, single-user systems where we want
automatic booting. To keep the system secure, the user should have
limited privileges.

Based on the service provided in the Cage wiki here:

https://github.com/Hjdskes/cage/wiki/Starting-Cage-on-boot-with-systemd



Co-Authored-By: Florian Klink <flokli@flokli.de>