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/quic.go

@@ -461,6 +461,9 @@ func (hrw *httpResponseAdapter) Header() http.Header {
 	return hrw.headers
 }
 
+// This is a no-op Flush because this adapter is over a quic.Stream and we don't need Flush here.
+func (hrw *httpResponseAdapter) Flush() {}
+
 func (hrw *httpResponseAdapter) WriteHeader(status int) {
 	hrw.WriteRespHeaders(status, hrw.headers)
 }