Make answer entity field-able

### Problem/Motivation
Quiz creator should be able to collect extra information, like attachments, audio recording, … without enable extra modules.

### Proposed resolution
Make result answer entity field-able.

### Remain tasks
- [x] Update entity info definition to provide UI to manage answer's fields.
- [x] Update answer form to attach fields.
- [x] Merge https://github.com/drupalprojects/quiz/commit/1bd5db7b3bdf7f7b4a551f07cb8fbefcf5c554d2
  - [ ] Provide a test case
- [x] save fields attached to form
- [x] Display custom fields in /quiz-result/%
- [x] test case