Function build_netinfo_cell 

pub(crate) fn build_netinfo_cell<S>(
    peer_ip: Option<IpAddr>,
    my_addrs: Vec<IpAddr>,
    sleep_prov: &S,
) -> Result<Netinfo>
where
    S: CoarseTimeProvider + SleepProvider,

Build a msg::Netinfo cell from the given peer IPs and our advertised addresses.

Both relay initiator and responder handshake use this.