Refactor/user and tests

[tmetzl]

This pull request refactors the user type used throughout the RBAC (Role-Based Access Control) system to use a new UserLike protocol instead of the concrete User class. This improves flexibility by allowing any object with the required attributes (username and groups) to be used as a user, and removes the dependency on the User class. The change is applied across the API, decorators, and documentation.

User type refactoring:

• Introduced the UserLike protocol in e2x_hub_rbac/auth/rbac.py, requiring username and groups attributes, and refactored all RBAC-related APIs and decorators to use UserLike instead of the User class. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

• Removed the User class definition from e2x_hub_rbac/auth/user.py, eliminating the dependency on Pydantic for user modeling.

Documentation updates:

• Updated all code examples and usage in README.md to use UserLike or a simple dataclass for users, instead of the old User class. [1] [2] [3] [4]

Bug fix:

• Fixed a bug in permission checking where term-scoped permissions now require both course_id and term_id in the context, ensuring correct validation.