Use Unix domain socket for the upstream

[alebedev87]

Currently the upstream can only be an URL to an http(s) endpoint which is bound to a network address and port. This would be nice to have a dedicated flag for the upstream which exposes a unix domain socket. The implementation would probably be quite simple: dedicated Dial function with unix as network argument in the transport.
The use case in which this can be useful: PODs which use hostnetwork - to minimize the number of ports occupied on the host.

[ibihim]

Hi, @alebedev87. We are not eager to push for new features, until we get the repository accepted into k8s/sig-auth.

[ibihim]

@alebedev87, I synced with the stakeholders and we would like to pause any new feature as long as we don't get accepted by k8s' sig-auth. I would keep this kind of open and come up with some "Backlog" label or so, so we don't forget about it.