API: Question Set - get question category distinct list

[TomasNiemczyk98]

Description

Modify existing model QuestionSetListItem to contain along with average difficulty for a question set also a list of distinct categories of questions in this set ordered by the count of their occurrences in this list

Cover new code with unit tests and E2E tests

User story

As an API consumer, I want to get with each question set also a list of categories from questions So that I can create a view like this:

AC

• [x] Extend the model to contain this data
• [x] Extend the use case to also get this data
• [x] All tests are passing