Several callbacks can be added to a same message type and disposed (#197)

[sticmac]

Related to #197.

This PR provides a solution which allows one to assign several callbacks to a same message type. Each OnMessage call returns an IDisposable object which can later be called through the Dispose method to remove the callback.

[lpsandaruwan]

Hi @endel,

I tested this, materials are in https://github.com/lpsandaruwan/colyseus-unity-sdk-test-pr-199 Looking good 👍 💯