Motivation for this change

Similarly to my motivation for #93598, I want to improve consistency between when networking.useNetworkd is true and when it's false. In particular I want to be able to write a NixOS module that can abstract out a non-trivial networking setup but still be agnostic to whether the system uses networkd or not.

Currently any routes that are set in networking.interfaces.<interface>.routes are completely ignored when using networkd. This commit instead adds them to the systemd.network.networks.<network>.routes option of the corresponding networkd network.

One thing that's tricky is that there isn't a trivial mapping between the options in networking.interfaces.<interface>.routes, which allows you to specify interface, via and options (the latter is passed more or less directly to the ip route command) and systemd.network's [Route] options. I've not attempted to do this in a complete way, but I do look at some of the options attributes and try to do the most sensible thing with them. I chose the particular options I did simply because they were useful to me. The "proper" way to do this would probably be to move them to be top level options of networking.interfaces.<interface>.routes rather than untyped strings in the options attrset..

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.