AsyncTask completion callback replaced by event

Changes
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1. Second parameter ```Action<AsyncTask> callback``` has been removed from ```AsyncTask``` constructor.
2. ```AsyncTaskCompletedHandler``` is introduced to replace the callback mechanism.
3. ```AsyncTask.Completed``` event has been introduced for task completion callback.

Testing
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- In the case of ```UpdateGraphAsyncTask```, verified that ```OnUpdateGraphCompleted``` is being called as expected.
- Test case ```AsyncTask00``` has been updated to reflect the signature change.

Hi @aparajit-pratap, please help taking a look. Thanks!