MINOR: quic: implement a basic "show quic" CLI handler

MINOR: quic: implement a basic "show quic" CLI handler

Implement a basic "show quic" CLI handler. This command will be useful
to display various information on all the active QUIC frontend
connections.

This work is heavily inspired by "show sess". Most notably, a global
list of quic_conn has been introduced to be able to loop over them. This
list is stored per thread in ha_thread_ctx.

Also add three CLI handlers for "show quic" in order to allocate and
free the command context. The dump handler runs on thread isolation.
Each quic_conn is referenced using a back-ref to handle deletion during
handler yielding.

For the moment, only a list of raw quic_conn pointers is displayed. The
handler will be completed over time with more information as needed.

This should be backported up to 2.7.