Motivation for this change

This PR consists of two changes:

• add `vrfConfig` option to netdevs: This change adds the VRF.Table option to netdev units in networkd. Although this affects just one option, I figured that this might still be helpful e.g. for accessibility and to point users to further resources about VRFs from our manual.

• add test for VRF configurations: This change adds a VM test which setups a small network where a VM communicates with other machines through a VRF. I decided to add this for two reasons:

  • As I didn't find too many resources about VRFs, I figured that this might be helpful for some people to get started (IMHO many VM-tests also serve as some kind of usage example).
  • On a Linux 5.4-kernel I had some issues with VRFs as it wasn't possible to SSH to an IP which is routed through the VRF. I think that such a test may help us to catch such regressions e.g. when updating the default kernel.

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.