Introduce 4.0 compatibility

[omares]

As I would like to revive this extension, this PR introduces compatibility with Akeneo 4.0.

This PR is intentionally a draft as there are a few todos that require feedback and in general, I would like to know if I am on the right track.

In the current state, the extension is installable, a new attribute of type "Text Collection" can be created and used. Interaction with the attribute is working (adding/deleting/sorting) and changes are persisted.

Open todos:

• [ ] need feedback Filtering: The attribute is selectable in the filters but the filter itself is not working. It seems that the attribute is not stored in elastic search, looking at the response of a direct request, the values key is empty.
• [ ] need feedback Is the pim_extended_attribute_type.denormalizer.flat.text_collection class still needed?
• [ ] need feedback Are there any open to-dos for PIMEE (workflows)?
• [ ] can do Remove commented code
• [ ] can do Fix tests
• [ ] can do Update Readme

[mmetayer]

I'll try to answer your need feedback questions today