add domain layer

[trunghvbk]

#1
Follow this article to add Domain Layer
https://codewithandrea.com/articles/flutter-app-architecture-domain-model/
Noted

Here are some tips you may use as you design and build your apps:

- explore the domain model and figure out what concepts and behaviors you need to represent
- express those concepts as entities along with their relationships
- implement the corresponding Dart model classes
- translate the behaviors into working code (business logic) that operates on those model classes
- add unit tests to verify the behaviors are implemented correctly
As you do that, think about what data you need to show in the UI and how the user will interact with it.