MINOR: muxes: Support an optional ALPN string when defining mux protocols

When a multiplexer protocol is defined, it is now possible to specify the
ALPN it supports, in binary format. This info is optionnal. For now only the
h2 and the h1 multiplexers define an ALPN because this will be mandatory for
a fix. But this could be used in future for different purpose.

This patch will be mandatory for the next fix.

include/haproxy/connection-t.h

@@ -670,6 +670,7 @@ struct mux_proto_list {
 	enum proto_proxy_mode mode;
 	enum proto_proxy_side side;
 	const struct mux_ops *mux;
+	const char *alpn;          /* Default alpn to set by default when the mux protocol is forced (optional, in binary form) */
 	struct list list;
 };