net/http: add Transport.Dialer, plumb RoundTrip contexts to net package

net/http: add Transport.Dialer, plumb RoundTrip contexts to net package

This simply connects the contexts, pushing them down the call stack.
Future CLs will utilize them.

For #12580 (http.Transport tracing/analytics)
Updates #13021

Change-Id: I5b2074d6eb1e87d79a767fc0609c84e7928d1a16
Reviewed-on: https://go-review.googlesource.com/22124
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>