Is there a way to Whitelist some IP addresses to prevent webhooks from being called by anybody ?
No sorry, nothing like this exists. If you need it IP address-based you would have to take care of that before a request reaches n8n, like in a reverse-proxy.
If you control the service that makes the request, the easiest solution would be adding authentication to the HTTP Request node. n8n checks then if the given user & password matches (Basic Auth) or a required header has the correct value (Header Auth).