MINOR: ssl: add the ssl_bc_sni sample fetch function to retrieve backend SNI

MINOR: ssl: add the ssl_bc_sni sample fetch function to retrieve backend SNI

Sometimes in order to debug certain difficult situations it can be useful
to know what SNI was configured on a connection going to a server, for
example to match it against what the server saw or to detect cases where
a server would route on SNI instead of Host. This sample fetch function
simply retrieves the SNI configured on the backend connection, if any.