Add option to choose between bird2 and frr

[felbinger]

It would be great to implement an option to use frr instead of bird2 for configuring DN42. If, like me, you're already running a router with bgp and want to add DN42 as a VRF on the same system, you're currently limited to using bird2, since running multiple BGP daemons on the same system is not recommended due to both needing to bind to port 179.

Maybe something like this for the option:

{
  options.networking.dn42.daemon = lib.mkOption {
    type = lib.types.types.enum [ "frr" "bird2" ];
    description = "Whether bird2 or frr should be used as daemon.";
    default = "bird2";
  };
}