Implement NAT discovery (RFC 5780) at the client

Snowflake clients will now attempt NAT discovery using the provided STUN
servers and report their NAT type to the Snowflake broker for matching.
The three possibilities for NAT types are:
- unknown (the client was unable to determine their NAT type),
- restricted (the client has a restrictive NAT and can only be paired
with unrestricted NATs)
- unrestricted (the client can be paired with any other NAT).

go.sum

@@ -64,6 +64,8 @@ github.com/pion/srtp v1.2.7 h1:UYyLs5MXwbFtXWduBA5+RUWhaEBX7GmetXDZSKP+uPM=
 github.com/pion/srtp v1.2.7/go.mod h1:KIgLSadhg/ioogO/LqIkRjZrwuJo0c9RvKIaGQj4Yew=
 github.com/pion/stun v0.3.3 h1:brYuPl9bN9w/VM7OdNzRSLoqsnwlyNvD9MVeJrHjDQw=
 github.com/pion/stun v0.3.3/go.mod h1:xrCld6XM+6GWDZdvjPlLMsTU21rNxnO6UO8XsAvHr/M=
+github.com/pion/stun v0.3.5 h1:uLUCBCkQby4S1cf6CGuR9QrVOKcvUwFeemaC865QHDg=
+github.com/pion/stun v0.3.5/go.mod h1:gDMim+47EeEtfWogA37n6qXZS88L5V6LqFcf+DZA2UA=
 github.com/pion/transport v0.6.0/go.mod h1:iWZ07doqOosSLMhZ+FXUTq+TamDoXSllxpbGcfkCmbE=
 github.com/pion/transport v0.8.10 h1:lTiobMEw2PG6BH/mgIVqTV2mBp/mPT+IJLaN8ZxgdHk=
 github.com/pion/transport v0.8.10/go.mod h1:tBmha/UCjpum5hqTWhfAEs3CO4/tHSg0MYRhSzR+CZ8=