MEDIUM: ssl: add sni support on the server lines

The new "sni" server directive takes a sample fetch expression and
uses its return value as a hostname sent as the TLS SNI extension.
A typical use case consists in forwarding the front connection's SNI
value to the server in a bridged HTTPS forwarder :

   sni ssl_fc_sni
diff --git a/include/types/server.h b/include/types/server.h
index 4b44f22..6a7e558 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -222,6 +222,7 @@
 		char *ca_file;			/* CAfile to use on verify */
 		char *crl_file;			/* CRLfile to use on verify */
 		char *client_crt;		/* client certificate to send */
+		struct sample_expr *sni;        /* sample expression for SNI */
 	} ssl_ctx;
 #endif
 	struct {