Issue619 - Provide method to unregister events

[RedOrc]

Problem

In Issue 619 it was reported by a user that there was no way to unregister events from the state machine. This leads to a memory leak in long running applications if you no longer want to be notified of transition or transition completed events.

Solution

Provide a way to unregister from both synchronous and asynchronous events. To successfully unregister callbacks, callers must maintain a reference to the original callback to ensure that it is cleared up. Otherwise, they can simply call stateMachine.UnregisterAllCallbacks() in order to remove all callbacks (no reference needed).

Documentation has been updated to reflect these new methods and the OnOffExample program has been tweaked to demonstrate registering and unregistering from callback events.

Changes

• Updated documentation to reflect new state machine API methods.
• Updated OnOffExample program to demonstrate the usage of the added APIs in the state machine.
• Added safety guards to prevent double subscription to the same callback method in both synchronous and asynchronous workflows.
• Added ability to Unregister from both synchronous and asynchronous workflows. Additional UnregisterAll method has been added to remove all callbacks at once with no reference pointer needed.
• Added new OnTransitionedEventTest.cs class to specifically test the OnTransitionedEvent.cs class. These tests provide coverage for the entire functionality of the transition event class.

[RedOrc]

@mclift I've got another one for you 🤠. It's a fix for the aforementioned issue in my description.

[mclift]

Thanks for preparing this PR!