[HTTPAPIs] Introduce a concrete type for the client `responseHandler`

Right now the `responseHandler` is a two parameter closure in the client. While this works, we know from previous API designs that this might limit future evolution since we can't add new parameters to the closure without breaking API. We should introduce a new structure `HTTPClientResponseReceiver` similar to the `HTTPServerResponseSender`. This will allow us to add more properties to the response sender in the future.