MEDIUM: quic: inspect ALPN to install app_ops

Remove the hardcoded initialization of h3 layer on mux init. Now the
ALPN is looked just after the SSL handshake. The app layer is then
installed if the ALPN negotiation returned a supported protocol.

This required to add a get_alpn on the ssl_quic layer which is just a
call to ssl_sock_get_alpn() from ssl_sock. This is mandatory to be able
to use conn_get_alpn().
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 8b81ef8..d53a387 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -6663,7 +6663,7 @@
  * freed by the caller. NPN is also checked if available since older versions
  * of openssl (1.0.1) which are more common in field only support this one.
  */
-static int ssl_sock_get_alpn(const struct connection *conn, void *xprt_ctx, const char **str, int *len)
+int ssl_sock_get_alpn(const struct connection *conn, void *xprt_ctx, const char **str, int *len)
 {
 #if defined(TLSEXT_TYPE_application_layer_protocol_negotiation) || \
 	defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)