Implement asynchronous authentication in QHttpNetworkConnection

This emits QHttpNetworkReply::authenticationRequired and expects a synchronous response.
Also proxyAuthenticationRequired when doing HTTP over a HTTP proxy.

QHttpNetworkReply needs a corresponding slot to be called when credentials are available.
This should be non configurable all the way up to QNetworkReplyHttpImpl, which for the backward compatibility code path should call back to the slot before returning.