net/http: introduce DialerAndTLSConfigSupportsHTTP2 in Transport

net/http: introduce DialerAndTLSConfigSupportsHTTP2 in Transport

Even when a custom TLS config or custom dialer is specified,
enables HTTP/2 if DialerAndTLSConfigSupportsHTTP2 is true.
By this change, avoid automatically enabling HTTP/2 if DialContext is set.

This change also ensures that DefaultTransport still automatically
enable HTTP/2 as discussed in #14391.

Updates #14391
Fixes #27011

Change-Id: Icc46416810bee61dbd65ebc96468335030b80573
Reviewed-on: https://go-review.googlesource.com/c/go/+/130256
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>