Support for LocalSocket in QNetworkAccessManager

Dockers REST api by default listens on a unix socket ( on mac / linux ) or a named pipe ( on windows ).

To allow Qt Creator ( and others ) to interface with this API we need a way to either directly support some form of url ( e.g. unix:///var/run/docker.socket ), or specify an alternative ( in this case QLocalSocket ) socket for QNAM to use for certain requests.

Trying to implement this currently is not feasible without re-implementing / copying the private HTTP Request classes. Using the QNetworkBackend mechanism is also not an option as it is private, and not used by the HTTP Request classes that QNAM uses.

As an example, see here for how the curl api supports this: https://github.com/cqbqdd11519/DockerClient/blob/master/docker.cpp#L238