http2: support ResponseController.EnableFullDuplex

The ResponseController.EnableFullDuplex method indicates that an HTTP
handler intends to interleave reads from a request body with writes
to the response body.

Add an EnableFullDuplex method to the ResponseWriter so we don't
return a not-supported error. The HTTP/2 server always supports
full duplex, so this is a no-op.

For golang/go#57786

Change-Id: I6529e6ce01d59b8b48fb67ba7c244255df57c174
Reviewed-on: https://go-review.googlesource.com/c/net/+/472717
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Дарья Бочкар <dasha7vanya@gmail.com>

http2/server.go

@@ -2880,6 +2880,11 @@ func (w *responseWriter) SetWriteDeadline(deadline time.Time) error {
 	return nil
 }
 
+func (w *responseWriter) EnableFullDuplex() error {
+	// We always support full duplex responses, so this is a no-op.
+	return nil
+}
+
 func (w *responseWriter) Flush() {
 	w.FlushError()
 }