CLEANUP: quic: create a dedicated quic_conn module

CLEANUP: quic: create a dedicated quic_conn module

xprt_quic module was too large and did not reflect the true architecture
by contrast to the other protocols in haproxy.

Extract code related to XPRT layer and keep it under xprt_quic module.
This code should only contains a simple API to communicate between QUIC
lower layer and connection/MUX.

The vast majority of the code has been moved into a new module named
quic_conn. This module is responsible to the implementation of QUIC
lower layer. Conceptually, it overlaps with TCP kernel implementation
when comparing QUIC and HTTP1/2 stacks of haproxy.

This should be backported up to 2.6.