MINOR: connection: Attach a "quic_conn" struct to "connection" struct.

This is a simple patch to prepare the integration of QUIC support to come.
quic_conn struct is supposed to embed any QUIC specific information for a QUIC
connection.
diff --git a/include/haproxy/connection-t.h b/include/haproxy/connection-t.h
index 4b1a21d..71272af 100644
--- a/include/haproxy/connection-t.h
+++ b/include/haproxy/connection-t.h
@@ -45,6 +45,7 @@
 struct server;
 struct session;
 struct pipe;
+struct quic_conn;
 
 /* Note: subscribing to these events is only valid after the caller has really
  * attempted to perform the operation, and failed to proceed or complete.
@@ -496,6 +497,7 @@
 	char *proxy_authority;	      /* Value of authority TLV received via PROXYv2 */
 	uint8_t proxy_authority_len;  /* Length of authority TLV received via PROXYv2 */
 	struct ist proxy_unique_id;  /* Value of the unique ID TLV received via PROXYv2 */
+	struct quic_conn *qc;         /* Only present if this connection is a QUIC one */
 };
 
 struct mux_proto_list {