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).

connection/http2.go

@@ -142,7 +142,7 @@ func (c *HTTP2Connection) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			break
 		}
 
-		rws := NewHTTPResponseReadWriterAcker(respWriter, r)
+		rws := NewHTTPResponseReadWriterAcker(respWriter, respWriter, r)
 		requestErr = originProxy.ProxyTCP(r.Context(), rws, &TCPRequest{
 			Dest:      host,
 			CFRay:     FindCfRayHeader(r),
@@ -289,6 +289,10 @@ func (rp *http2RespWriter) Header() http.Header {
 	return rp.respHeaders
 }
 
+func (rp *http2RespWriter) Flush() {
+	rp.flusher.Flush()
+}
+
 func (rp *http2RespWriter) WriteHeader(status int) {
 	if rp.hijacked() {
 		rp.log.Warn().Msg("WriteHeader after hijack")