MINOR: ssl: add ssl_fc_unique_id to fetch TLS Unique ID

The TLS unique id, or unique channel binding, is a byte string that can be
pulled from a TLS connection and it is unique to that connection. It is
defined in RFC 5929 section 3.  The value is used by various upper layer
protocols as part of an extra layer of security.  For example XMPP
(RFC 6120) and EST (RFC 7030).

Add the ssl_fc_unique_id keyword and corresponding sample fetch method.
Value is retrieved from OpenSSL and base64 encoded as described in RFC
5929 section 3.