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.
Correct, however it seems as though there were issues in the past with the default gateway updates through networking-setup.service not always being triggered. Is there a better system for triggering these updates as opposed to the scripted system we have now. Is there some way we could use bindTo or some similar systemd unit descriptor to trigger the reloads?