nixos/cloudflare-dyndns: introduce package option

    services.cloudflare-dyndns = {

      enable = mkEnableOption "Cloudflare Dynamic DNS Client";

      package = mkPackageOption pkgs "cloudflare-dyndns" { };

      apiTokenFile = mkOption {

        type = types.nullOr types.str;

        default = null;

              ++ optional cfg.deleteMissing "--delete-missing"

              ++ optional cfg.proxied "--proxied";

          in

          "${pkgs.cloudflare-dyndns}/bin/cloudflare-dyndns ${toString args}";

          "${getExe cfg.package} ${toString args}";

      };

    } // optionalAttrs (cfg.frequency != null) {

      startAt = cfg.frequency;