net/http: add ResponseController and per-handler timeouts

net/http: add ResponseController and per-handler timeouts

The ResponseController type provides a discoverable interface
to optional methods implemented by ResponseWriters.

	c := http.NewResponseController(w)
	c.Flush()

vs.

	if f, ok := w.(http.Flusher); ok {
		f.Flush()
	}

Add the ability to control per-request read and write deadlines
via the ResponseController SetReadDeadline and SetWriteDeadline
methods.

For #54136

Change-Id: I3f97de60d4c9ff150cda559ef86c6620eee665d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/436890
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>