Add utility to help user discover their NAT type

[maxhawkins]

Summary

Other tools like go-stun can print the type of NAT detected by the STUN server:

> go-stun
NAT Type: Full cone NAT
External IP Family: 1
External IP: 166.111.4.100
External Port: 23009

t would be convenient if our stun did this too, and printed the result to the info log. That way people could include it in issue descriptions while debugging ICE.

[backkem]

Note; See if we can detect if ´CHANGE-ADDRESS´ is supported.

[Sean-Der]

This is done! @cohosh added a really cool utility that prints the mapping and filtering type of your NAT

2020/05/29 12:48:37 Connecting to STUN server: stun.voip.blackberry.com:3478
2020/05/29 12:48:37 Local address: 0.0.0.0:46068
2020/05/29 12:48:37 Received xormapped address: 23.252.60.236:46068
2020/05/29 12:48:37 Received xormapped address: 23.252.60.236:46068
2020/05/29 12:48:37 NAT mapping behavior: endpoint-independent
2020/05/29 12:48:37 Local address: 0.0.0.0:60158
2020/05/29 12:48:37 Received xormapped address: 23.252.60.236:60158
2020/05/29 12:49:07 NAT filtering behavior: address-dependent

[introspection3]

@Sean-Der how write this code