[WIP] Comments on product

| 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               |
We have a getId() on product but the `CommentBundle` doesn't know it (and should not know it). Yes it makes `CatalogBundle` dependent to `CommentBundle`, but this is the right direction of this dependency. And `CommentBundle` should not handle any kind of objects. That's why this `CommentSubjectInterface` is here.
The best would be to have : `CatalogBundle` and `CommentBunde` totally independent. All "glue" in a "CoreBundle" : real Core products should implements `CommentSubjectInterface` and  `ProductInterface`.