MINOR: quic-sock: provide a pair of get_src/get_dst functions

MINOR: quic-sock: provide a pair of get_src/get_dst functions

These functions will allow the connection layer to retrieve a quic_conn's
source or destination when possible. The quic_conn holds the peer's address
but not the local one, and the sockets API doesn't always makes that easy
for datagrams. Thus for frontend connection what we're doing here is to
retrieve the listener's address when the destination address is desired.

Now it finally becomes possible to fetch the source and destination using
"src" and "dst", and to pass an incoming connection's endpoints via the
proxy protocol.