client: add sidechannel support

Implements gitlab-com/gl-infra/scalability#1303.

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.

client/dial_test.go

@@ -218,10 +218,6 @@ func TestDialSidechannel(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			if emitProxyWarning() {
-				t.Log("WARNING. Proxy configuration detected from environment settings. This test failure may be related to proxy configuration. Please process with caution")
-			}
-
 			if tt.envSSLCertFile != "" {
 				defer testhelper.ModifyEnvironment(t, gitalyx509.SSLCertFile, tt.envSSLCertFile)()
 			}