nixos/network-interfaces: Implement the ability to add arbitrary routes

This is currently a work in progress as supporting default routes is probably not quite working. I'd appreciate testers when this code actually evaluates.
Maybe we can have a udev action triggered whenever an interface is added, which would call "ip route add default ...".

Or maybe we should look into systemd-network, which might take care of this for us in a presumably udev-friendly way.