net/http: support BaseContext & ConnContext for http2 Server

This is the net/http half of #32476. This supplies the method needed
by the other half in x/net/http2 in the already-submitted CL 181259,
which this CL also bundles in h2_bundle.go.

Thanks to Tom Thorogood (@tmthrgd) for the bug report and test.

Fixes #32476
Updates #30694

Change-Id: I79d2a280e486fbf75d116f6695fd3abb61278765
Reviewed-on: https://go-review.googlesource.com/c/go/+/181260


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>