Expose flat API for ISteamNetworkingMessages

[copyrat90]

I've tested calling these new flat APIs with a test commit in my C# binding GnsSharp:

SteamNetworkingIPAddr addr = default;
addr.ParseString("127.0.0.1:43000");

SteamNetworkingIdentity ident = default;
ident.SetIPAddr(in addr);

ISteamNetworkingMessages.User!.SendMessageToUser(ident, data, ESteamNetworkingSendType.ReliableNoNagle, 0);

And the flat API is successfully called, at least:

[Verbose] Messages session ip:127.0.0.1:43000: created
[Bug] Cannot use P2P connectivity.  CreateConnectionSignaling callback not set
[Bug] src\steamnetworkingsockets\clientlib\csteamnetworkingmessages.cpp(473): Failed to create connection to 'ip:127.0.0.1:43000' for new messages session