Inspired from the dhcpd service implementation
Only 2 configurations options at the moment:
enabled and path to configfile

folder networking has been chosen because radius is mainly used for network device and network authentication (e.g. WPA2 Entreprise)

Motivation for this change

Freeradius package is already supported by nix, but no service exists for nixos. This patch adds basic support to start the freeradius service.

Things done

I tested the service configuration in my personal nix installation successfully.

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.