QNetworkAccessManager fails to authenticate if proxy has username & password

My initial investigation was flawed, for a simple squid based proxy everything works as expected. It seems to only be a problem for a certain customer behind an NTLM proxy: QTCREATORBUG-30369

When a proxy is configured on macOS, the username and password cannot be retrieved by the functions used by Qt at the moment (CFNetworkCopySystemProxySettings).

This means that every Qt application has to implement QNetworkAccessManager::proxyAuthenticationRequired which is non-trivial to do well.

Since the user has configured username & password, Qt should also use that automatically.
I'm aware that this is a huge amount of effort.