Make sharing local IP and Bluetooth address optional
If you make an introduction/discovery of a new contact, you'll each share Bluetooth address and local IP with each other. (See also: https://chat.briarproject.org/briar/pl/ygqns71cjiym8kjzfc4jyfywgw)
While this is needed to communicate offline, there are use cases that don't need it. OptOut/In to share such info during contact discovery could make sure to only share that data if it seem reasonable.