(api) Make code tidier by replacing scopes as strings

## Feature Request

Scopes introduced in `user.py` (https://github.com/openfun/ralph/blob/34c8413efc0cdd1bf698974318677762151f7e2d/src/ralph/api/auth/user.py) by #463 are hard coded strings. It has been suggested to replace this by Enum or pydantic models.
