Allow users to unpack messages too

[paulpach]

Provide a convenient method for unpacking messages. This is useful for OnServerAddPlayer.

This is how you call AddPlayer:

class Credentials : MessageBase
{
     public string username;
     public string password;
}

// now add a player
Credentials credentials = new Credentials() 
{
      username = "Joe",
      password = "Gabagaba"
};
client.AddPlayer(connection,  MessagePacker.pack(credentials));


// and this is how you receive it:
public override void OnServerAddPlayer(NetworkConnection conn, AddPlayerMessage extraMessage)
{
    Credentials credentials = MessagePacker.Unpack<Credentials>(extraMessage.value);
    ...
}

This PR also adds a unit test for packing/unpacking

[miwarnec]

🎉 This PR is included in version 1.0.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀