Register a callback function for the session that will be called
when a TLS Finished message has been generated. The function is
typically used to copy away the TLS finished message for later use
as a channel binding or similar purpose.
The finished parameter will contain the binary TLS finished
message, and len will contains its length. For SSLv3 connections,
the len parameter will be 36 and for TLS connections it will be
12.
It is recommended that the function returns quickly in order to not
delay the handshake. Use the function to store a copy of the TLS
finished message for later use.