feature request: add sent_packets field to noq-proto PathStats and ConnectionStats

[Sherlock-Holo]

there is a sent_packets in https://docs.rs/quinn/latest/quinn/struct.PathStats.html

with sent_packets and lost_packets data, we can calculate the loss rate, however, the noq-proto miss sent_packets

[flub]

FWIW, it's good to add this but you can already mostly do this loss rate calculation by using UdpStats::datagrams from the PathStats::udp_tx. Only during the handshake are a few packets coalesced into the same datagram and might you encounter a small discrepancy.

[mvanhorn]

Implemented this in #696.