MINOR: mux-quic: define is_active app-ops

MINOR: mux-quic: define is_active app-ops

Add a new app layer operation is_active. This can be used by the MUX to
check if the connection can be considered as active or not. This is used
inside qcc_is_dead as a first check.

For example on HTTP/3, if there is at least one bidir client stream
opened the connection is active. This explicitly ignore the uni streams
used for control and qpack as they can never be closed during the
connection lifetime.