crypto/tls: add Config.VerifyConnection callback

crypto/tls: add Config.VerifyConnection callback

Since the ConnectionState will now be available during
verification, some code was moved around in order to
initialize and make available as much of the fields on
Conn as possible before the ConnectionState is verified.

Fixes #36736

Change-Id: I0e3efa97565ead7de5c48bb8a87e3ea54fbde140
Reviewed-on: https://go-review.googlesource.com/c/go/+/229122
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>