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 provide a notification to 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]).

The app will get notified via on_call_media_transport_state() callback
and it can inspect ext_info to verify the operation to be equal to
PJ_ICE_STRANS_OP_ADDR_CHANGE.

[ming]

In 5282:

Fixed #1913: Add callback for address change notification from STUN keep alive