client: add sidechannel support
Implements gitlab-com/gl-infra/scalability#1303 (closed).
This change adds publicly exported code that allows Gitaly clients to accept sidechannel connections coming back from a Gitaly server they have connected to. This is done via a new dial function DialSidechannel.
We need this code to be public so that Workhorse can import it. Without this code, Workhorse won't be able to use sidechannel RPC's.
Edited by Jacob Vosmaer