MINOR: quic: dump quic_conn debug string for logs

MINOR: quic: dump quic_conn debug string for logs

Define a new xprt_ops callback named dump_info. This can be used to
extend MUX debug string with infos from the lower layer.

Implement dump_info for QUIC stack. For now, only minimal info are
reported : bytes in flight and size of the sending window. This should
allow to detect if the congestion controller is fine. These info are
reported via QUIC MUX debug string sample.

(cherry picked from commit 663416b4ef2fafcffa61c04aa09056853d9674f7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>