Fix: Support Gluetun widget auth (#4034)

docs/widgets/services/gluetun.md

@@ -11,8 +11,11 @@ Learn more about [Gluetun](https://github.com/qdm12/gluetun).
 
 Allowed fields: `["public_ip", "region", "country"]`.
 
+To setup authentication, follow [the official Gluetun documentation](https://github.com/qdm12/gluetun-wiki/blob/main/setup/advanced/control-server.md#authentication).
+
 ```yaml
 widget:
   type: gluetun
   url: http://gluetun.host.or.ip:port
+  key: gluetunkey # Not required if /v1/publicip/ip endpoint is configured with `auth = none`
 ```