Enable entityreference filters on product attributes [#3441649]

Problem/Motivation

It is possible to add a taxonomy term linked to a product (or variation) as an entityreference filter for product type (or variation type), but it seems that the same behaviour is not possible with product attributes used in the variations.

Steps to reproduce

Create a views of variations
Add the Relationships: Product variation using attribute_ your attribute
Try to add your attribute as filter: it does not appear in the list as entityreference filter

Proposed resolution

I made the following merge request

Issue fork entityreference_filter-3441649

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3441649-enable-entityreference-filters changes, plain diff MR !23
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