nixos/serviio: add option to open firewall ports

I think we should have open firewall ports and let the user choose if they want to have these ports open or not. :D

nixos/modules/services/misc/serviio.nix

···
         '';
       };
 
+      openFirewall = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Open ports in the firewall for the Serviio Media Server.
+        '';
+      };
+
       dataDir = lib.mkOption {
         type = lib.types.path;
         default = "/var/lib/serviio";
···
 
     users.groups.serviio = { };
 
-    networking.firewall = {
+    networking.firewall = lib.mkIf cfg.openFirewall {
       allowedTCPPorts = [
         8895 # serve UPnP responses
         23423 # console