MINOR: quic: refactor qc_new_conn() prototype

MINOR: quic: refactor qc_new_conn() prototype

The objective of this patch is to streamline qc_new_conn() usage so that
it is similar for frontend and backend sides.

Previously, several parameters were set only for frontend connections.
These arguments are replaced by a single quic_rx_packet argument, which
represents the INITIAL packet triggering the connection allocation on
the server side. For a QUIC client endpoint, it remains NULL. This usage
is consider more explicit.

As a minor change, <target> is moved as the first argument of the
function. This is considered useful as this argument determines whether
the connection is a frontend or backend entry.

Along with these changes, qc_new_conn() documentation has been reworded
so that it is now up-to-date with the newest usage.