crypto/tls: add SessionState.Extra

crypto/tls: add SessionState.Extra

This can be used by applications to store additional data in a session.

Fixes #57753
For #60105

Change-Id: Ib42387ad64750fa8dbbdf51de5e9c86378bef0ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/496822
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Marten Seemann <martenseemann@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>