ssl-utils: add signed_certificate_timestamp tls hello extension

This extension is defined in RFC 6962.

Change-Id: I3aa7321c60baef59ccb59ded6b91f3e42c854bfa
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://code.wireshark.org/review/970
Reviewed-by: Michael Mann <mmann78@netscape.net>