Investigate possible `User` domain model

[sokimaaa]

Context

• There is no model for User Service.
• Need to take into account multiple aspects of what model is needed for kick-off.
• Discuss with the team to take into account their perspectives.
• Need to consider the possibility of extending the current model in the future, for example by extending to OrderV02.
• The system needs at least representations of User accounts, roles (merchant, delivery management, customer, and so on), and user observation info (which products are visited or interested in the certain user) to base recommendations on this data.

Acceptance Criteria

• The created V01 model for User Service where took into account the requirements.
• JavaDoc presence
• Considered future extending of the model (Generics, method overloading/overriding, deprecation strategy)
• Fully compliant with other models
• Updated to V02 other models that need user info
• Style consistent with other models

Technical Details

• Explore the E-commerce business perspective
• Explore ways to extend the model interface, e.g. Generics, method overloading, overriding, deprecation strategy