feat: Implement `User` model, service and repository

As a developer, I want to have a place to interact with the user data and functions

Requirements

You can find the user definition here

Implement a repository that:

Use freezed (and Isar if user is stored in the database) to implement the User, and UserPreferences models
Create a UserDataLoader class that:
- synchronize the user with the API (depends on API #16)
Create a UserService class that has the following function:
- retrieve the user from the database or shared preferences
- Future<bool> updateUser(User updatedUser) which update the locally stored user with a new one then synchronize with the API if necessary ((depends on API #15)
- placeholder functions (throw UnimplementedError()) for:
- Future<bool> deleteUser(User user);
Create an enum AuthenticationMethod with the values: google, apple, classic, anonymous
Create a UserRepository class as a listenable service, and add it to the locator. It should have the following methods:
- getter for User self that returns the user connected (registered as listenToReactiveValues)
- Future<bool> signOut() which clear the current user
- placeholder functions (throw UnimplementedError()) for:
- Future<bool> signIn(AuthenticationMethod method, {bool isSilent = false, String? email, String? password})
- Future<bool> register()
- Future<bool> updateSelf(User updatedUser);
- Future<bool> linkAccount(AuthenticationMethod method, {String? email, String? password})
- Future<bool> deleteUser();
- Future<bool> updatePassword(String oldPassword, String newPassword)
Update the locator to add the UserRepository and UserDataLoader

Child of #130

Depends on: