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.

(cherry picked from commit dae4cfe8c57dc7644a7afd847d964c8d7444deda)
[wt: backported in order to help tracking some issues]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit ca6a7b4635ab9417e4f8a171b28269c145e1da33)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a60a266393ad52b3f9e50e96348e7970ba9d17fd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>