TUN-7558: Flush on Writes for StreamBasedOriginProxy

In the streambased origin proxy flow (example ssh over access), there is
a chance when we do not flush on http.ResponseWriter writes. This PR
guarantees that the response writer passed to proxy stream has a flusher
embedded after writes. This means we write much more often back to the
ResponseWriter and are not waiting. Note, this is only something we do
when proxyHTTP-ing to a StreamBasedOriginProxy because we do not want to
have situations where we are not sending information that is needed by
the other side (eyeball).

orchestration/orchestrator_test.go

@@ -450,7 +450,7 @@ func proxyTCP(ctx context.Context, originProxy connection.OriginProxy, originAdd
 		CFRay:   "123",
 		LBProbe: false,
 	}
-	rws := connection.NewHTTPResponseReadWriterAcker(respWriter, req)
+	rws := connection.NewHTTPResponseReadWriterAcker(respWriter, w.(http.Flusher), req)
 
 	return originProxy.ProxyTCP(ctx, rws, tcpReq)
 }