Implement a basic TLS 1.3 client handshake, only enabled if explicitly
requested with MaxVersion.

This CL intentionally leaves for future CLs:
  - PSK modes and resumption
  - client authentication
  - post-handshake messages
  - downgrade protection
  - KeyLogWriter support

Updates #9671

Change-Id: Ieb6130fb6f25aea4f0d39e3a2448dfc942e1de7a
Reviewed-on: https://go-review.googlesource.com/c/146559
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>