feature(backend): implement GET `/api/quizzes/{objectId}` endpoint

[acsolle66]

The /api/quizzes/{objectId} endpoint should return a quiz object or 404 if not found. To reach the goal of this issue, the following tasks need to be done:

• [x] add retrieve_quiz function to quizapp/backend/quizapp/crud.py
• [x] add get_quiz endpoint function to quizapp/backend/quizapp/routers/quizzes.py with proper error handling of quiz if it is not in the database
• [x] if the test database is set up by the time you're working on this issue, please add tests to your endpoint.

For guidance:

• check this tutorial Building a CRUD App with FastAPI and MongoDB
• Motor docs
• FastAPI docs