Add callback for address change notification from STUN keep alive

[ming]

From our IP Address Change wiki:
"If ICE media transport is used, and STUN is enabled on the media transport, then the media transport will automatically update its publicly mapped IP address from the STUN Binding response. The transport should send STUN Binding request periodically (approximately every 15 seconds) as NAT keep-alive mechanism, so the address change will be detected by the transport automatically during this operation."

This ticket is to add a callback to notify the app when such address change happens due to the STUN keep alive operation (as indicated by the log saying: STUN mapped address found/changed: [new address here]).