add presentation layer sample

[trunghvbk]

#1
Referred articles: https://codewithandrea.com/articles/flutter-presentation-layer/

By implementing a custom controller class based on StateNotifier, we've separated our business logic from the UI code.

As a result, our widget class is now completely stateless and is only concerned with:

watching state changes and rebuilding as a result (with ref.watch)
responding to user input by calling methods in the controller (with ref.read)
listening to state changes and showing errors if something goes wrong (with ref.listen)
Meanwhile, the job of our controller is to:

talk to the repository on behalf of the widget
emit state changes as needed
And since the controller doesn't depend on any UI code, it can be easily unit tested.

This makes it an ideal place to store any widget-specific business logic.