We now depend on libsodium unconditionally. Future work will require
functions from libsodium, and nobody we're aware of uses the nacl build
for anything other than making sure it still works on CI.
This function will return an IP address string associated with a peer.
If the peer is not accepting direct connections a placeholder value
will be returned, indicating that their real IP address is unknown.
We do not return TCP relay IP addresses because a TCP connection
with a peer may use multiple relays simultaneously.
