feat: Implement `Achievement` model, repository, and service

[apomalyn]

Requirements

You can find the Achievement definition here

Implement a service that:

• retrieve the list of achievements from the database
• sync the list with the API

Implement a repository that:

• retrieve the list of achievements from the service
• keep it in memory

What needs to be done

• Using freezed and Isar, implement the Achievement model
• Implement a AppEvents enum with the following values:
  • quiz.started
  • quiz.answered
  • quiz.ended
• Implement an AchievementsService, it should have the following methods:
  • List<Achievement> getAll() retrieve all the achievements from the database
  • Future<void> synchronize() which calls the API and synchronizes the local list with the API (depends on API #25)
• Update the splash view-model to synchronize the achievements list between the API and the application
• Implement an AchievementsRepository as a listenable service, and register it on the locator. It should have the following methods:
  • getter for List<Achievement> achievements that load from the service all the achievements if not already loaded
  • registered as listenToReactiveValues

Dependencies

Child of #31

Depends on:

• API #25