MEDIUM: quic/mux-quic: adjust app-ops install

MEDIUM: quic/mux-quic: adjust app-ops install

This patch reworks the installation of app-ops layer by QUIC MUX.
Previously, app_ops field was stored directly into the quic_conn
structure. Then the MUX reused it directly during its qmux_init().

This patch removes app_ops field from quic_conn and replaces it with a
copy of the negotiated ALPN. By using quic_alpn_to_app_ops(), it ensures
it remains compatible with a known application layer.

On the MUX layer, qcc_install_app_ops() now uses the standard
conn_get_alpn() to retrieve the ALPN from the transport layer. This is
done via the newly defined <get_alpn> QUIC xprt callback.

This new architecture should be cleaner as it better highlights the
responsibility of each layers in the ALPN/app negotiation.