Support stand-alone GameNetworkingSockets

[copyrat90]

Currently, Backdash.Gns only supports Steamworks SDK.

It is trivial to add support for the open source stand-alone GameNetworkingSockets, but there are several blockers on the GNS upstream:

• P2P support is broken
  • test_p2p is not working when trivial_signaling_server is remote. ValveSoftware/GameNetworkingSockets#247
  • P2P Not Working, test_p2p.exe Not Working ValveSoftware/GameNetworkingSockets#354
  • test_p2p.py (local P2P test) fails even on non-symmetric mode ValveSoftware/GameNetworkingSockets#362
• Stand-alone GNS doesn't expose ISteamNetworkingMessages
  • Expose flat API for ISteamNetworkingMessages ValveSoftware/GameNetworkingSockets#368

Even if the support is implemented, it is not possible to use it until all of these are fixed.