Add an API function to get the IP address for a friend

[iphydf]

Toxcore knows this. It would be useful to be able to show this to the users as well, e.g. for debugging or to see whether the tor proxying is working. When connected via TCP relay, we should probably show the TCP relay's address (which is also useful for debugging).

There's no reason not to expose this information other than that it widens the API. We should put it into tox_private.h for now, because we may want to change what that API looks like. Over time, I'd like to see something like a tox_debug.h which has all kinds of insights into toxcore inner details.