[POC] Feature variant groups

Very first POC on variant groups enhancement, store product template and apply on products

Process Edit from UI,
 - postLoad : from DB, [] -> denormalize(json) -> ProductValueInterface[]
 - set the ProductValueInterface in the form

Process Save from UI
 - ProductValueInterface[] are binded in the Form
 - ProductValueInterface are validated
 - prePersist, preUpdate : ProductValueInterface[] -> normalize(json) -> []
 - Save the VariantGroup

Process Apply values on products,
 - from DB, from valuesData, [] is json format, can be applied with ProductUpdater
 - from DB, from values objects, ProductValueInterface[] -> normalize(json) -> [] -> ProductUpdater

Process Export variant groups
 - introduce a Group Processor, if is_variant, fetch the product template
 - from the template get the product values objects
 - use the csv normalizer to normalize the values

Process Import variant groups
 - introduce a Group Reader, if is_variant, get the values data
 - from the values [] uses the csv denormalizer to get values object
 - validate the values objects
 - set the values in the template (normalized by the prePersist / preUpdate)

Notes,
 - prePersist, preUpdate, postLoad can be performance killers
 - perhaps use a more explicit way to load the product template values
 - or don't load by default the product template of a product, they'll be lazy loaded if getProductTemplate is called

Questions,
 - Add a dedicated import / export for the variant group product template ?
 - Run the copy in background with a command ? (be able to run with ajax request from the UI)

TODO,
 - introduce a GroupInterface in Model and use it everywhere
 - introduce a GroupProductTemplateInterface in Model and use it everywhere
 - add resolve target for these entities
 - implement the product value denormalizer from csv format
 - implement the product value denormalizer from json format
 - re-work versioning denormalizer (EE)

| 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               |
@filips we target to use json format here (normalizer already exists)