Add custom field filters to epics list page

What does this MR do and why?

Filter epics by custom fields

#525462 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable custom_fields_feature flag
2. Add a Single or Multi select custom field for a top level group, add Epic work item type to it
3. Go to Epic list list page
4. Filter by the custom field

MR acceptance checklist

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