PIM-3530 : add variant group values denormalizer (flat/csv)

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | Y
| BC breaks?           | N
| CI currently passes? | ?
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | N
| Fixed tickets        | PIM-3530 (partially)
| DB schema updated?   |
| Migration script?    |
| Doc PR               |