net/http: allow configuring read timeout of response body with transport

[nhooyr]

I'd like ResponseHeaderTimeout but for the response body.

[meirf]

I commented on your other issue here: #26942 (comment). If you have the same response, let's just keep the convo there. Otherwise, could be better to have separate conversations.

[nhooyr]

Actually this should go into http.Transport.

[nhooyr]

Just realized I can just cancel the request context to make this work. All is good.