[WIP] PIM-3530 : variant groups, re-work csv denormalizers

Some data denormalization doesnt work when we denormalize from a single value (and not from the whole product)
 - [x] metric
 - [x] prices
 - [ ] media

| Q                    | A
| -------------------- | ---
| Bug fix?             |
| New feature?         |
| BC breaks?           |
| CI currently passes? |
| Tests pass?          |
| Scenarios pass?      |
| Checkstyle issues?*  |
| PMD issues?**        |
| Changelog updated?   |
| Fixed tickets        |
| DB schema updated?   |
| Migration script?    |
| Doc PR               |