http2: support net/http.Transport.NewClientConn

Permit net/http to create new HTTP/2 client connections.

We do this by adding a NewClientConn method to the type the HTTP/2 client
registers with net/http.Transport.RegisterProtocol, which creates a
persistent connection from a net.Conn.

No tests in this CL. Tests will be in net/http, and will cover
both the HTTP/1 and HTTP/2 paths for NewClientConn.

For golang/go#75772

Change-Id: Ib1a06b4d13fdd6008e5db9a090c6e9632029a2a4
Reviewed-on: https://go-review.googlesource.com/c/net/+/722200
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>