feat(nixos): wireguard add config with agenix

[storopoli]

Use agenix to create a wireguard kill switch config:

• [ ] Encode secrets with agenix

  environment.etc = {
    # wireguard config used with `wg-quick up wg-example`
    "wireguard/wg-example.conf" = {
      source = config.age.secrets."wg-example.conf".path;
  };

• [ ] Create the config file wg-something.conf with systemd-networkd

• [ ] Replace in-place strings in wg-something.conf with secrets

• [ ] Setup the systemd autostart