Centralized Callback Manager

[momintlh]

• Added CallbackManager.cs which will replace all of the Dictionaries/Lists we are using for handling callbacks for different parts of playroom

• Currently tested with InsertCoin's callbacks, which does not require any parameters: Type Action (void)

TODO:

• [x] Test with other types: Action< string > (Tested and working with WaitForState)
• [x] Action<string, string>
• [ ] Test with persistent callbacks (for RPCs)

Based On #75 Relates to #40

[SaadBazaz]

We should be pushing out a callback manager which we can use for WaitForState, solving the original problem in #75 . If we implement WaitForState with this new CallbackManager, we can keep improving it later on and incrementally replace other callbacks.