Enhance customer group filter in rule engine

[pfferrari]

Closes commercelayer/issues-app/issues/631

What I did

Enhanced customer_group filtering in Rule Engine by treating this kind of resource in a way to render a selectable filter, instead of a manual text one. This behavior was already present for other resources like markets, skus and few others.

Thanks to this when I filter by customer group (previously named customer group id) I can select the wanted customer group in searchable select and obtain the same result of manually typing the customer resource id in the old manual text filter.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.

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