MINOR: quic: Add spin bit support

MINOR: quic: Add spin bit support

Add QUIC_FL_RX_PACKET_SPIN_BIT new RX packet flag to mark an RX packet as having
the spin bit set. Idem for the connection with QUIC_FL_CONN_SPIN_BIT flag.
Implement qc_handle_spin_bit() to set/unset QUIC_FL_CONN_SPIN_BIT for the connection
as soon as a packet number could be deciphered.
Modify quic_build_packet_short_header() to set the spin bit when building
a short packet header.

Validated by quic-tracker spin bit test.

Must be backported to 2.7.