[WIP] Kaf notifications

| Q                    | A
| -------------------- | ---
| Bug fix?             |
| New feature?         |
| BC breaks?           |
| CI currently passes? |
| Tests pass?          |
| Scenarios pass?      |
| Checkstyle issues?*  |
| PMD issues?**        |
| Changelog updated?   |
| Fixed tickets        |
| Doc PR               |
I'm really surprised by the term NotificationEvent, with a such name I expect to see a dispatched model, not a stored entity. Let's discuss about this naming :)