PIM-3026: Dispatch post create event for attribute group

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | TODO
| Fixed tickets        | PIM-3026
| Doc PR               |
always wondering if it's a good idea to put such classes as final
maybe abstract is enough