dns/ddclient - allow custom target hostname for dyndns2 protocol. closes https://github.com/opnsense/plugins/issues/3362

It looks like the backend code was already available, we just forgot to broadcast it to the frontend using the known_services() method

dns/ddclient/src/opnsense/scripts/ddclient/lib/account/dyndns2.py

@@ -53,7 +53,7 @@ class DynDNS2(BaseAccount):
 
     @staticmethod
     def known_services():
-        return  DynDNS2._services.keys()
+        return  list(DynDNS2._services.keys()) + ['custom']
 
     def match(account):
         if account.get('service') in DynDNS2._services or (