MINOR: quic: Add information to "show quic" for CUBIC cc.

MINOR: quic: Add information to "show quic" for CUBIC cc.

Add ->state_cli() new callback to quic_cc_algo struct to define a
function called by the "show quic (cc|full)" commands to dump some information
about the congestion algorithm internal state currently in use by the QUIC
connections.

Implement this callback for CUBIC algorithm to dump its internal variables:
   - K: (the time to reach the cubic curve inflexion point),
   - last_w_max: the last maximum window value reached before intering
     the last recovery period. This is also the window value at the
     inflexion point of the cubic curve,
   - wdiff: the difference between the current window value and last_w_max.
     So negative before the inflexion point, and positive after.