MINOR: mux_quic: define ms_bidi_rel QCC member

MINOR: mux_quic: define ms_bidi_rel QCC member

Add a new QCC member <ms_bidi_rel>. This represents the number of
concurrent streams advertised similarly to ms_bidi, but as a relative
value.

This patch does not introduce any functional change. For now,
<ms_bidi_rel> will be equal to <ms_bidi_init>. However, with the
implementation of stream elasticity and dynamic adjustment for
concurrent max-streams-bidi, the former will be required to keep the
last advertised value.