Automatic merge from submit-queue

Extensible Userspace Proxy

This PR refactors the userspace proxy to allow for custom proxy socket implementations.
It changes the the ProxySocket interface to ensure that other packages can properly implement it (making sure all arguments are publicly exposed types, etc), and adds in a mechanism for an implementation to create an instance of the userspace proxy with a non-standard ProxySocket.
Custom ProxySockets are useful to inject additional logic into the actual proxying.  For example, our idling proxier uses a custom proxy socket to hold connections and notify the cluster that idled scalable resources need to be woken up.

Also-Authored-By: Ben Bennett bbennett@redhat.com