Load unified epic and epic work item labels

What does this MR do and why?

re Inconsistent labels returned over the api when ... (#479317 - closed)

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Before	After
Screen_Recording_2024-08-21_at_12.52.23	Screen_Recording_2024-08-21_at_12.02.05

How to set up and validate locally

• Pre-requisites
  • Make sure to have following FFs enabled: work_item_epics, work_item_epics_rollout, namespace_level_work_items, epic_and_work_item_associations_unification
  • Create a couple epics and related them to one another
  • Access one of the epics details page and set some label(s). If above FFs are enabled this would set the labels on the Epic Work Item side
  • Access the same epic but add force_legacy_view=true parameter to the URL and add some other label(s), this would access the legacy epic view and would set the labels on the Legacy Epic object. You should still see all the labels on the epic details page
• Test steps
  • On master
    • Access following endpoints on master branch and then on this branch and look for labels attribute:
      • api/v4/groups/491/epics/
      • api/v4/groups/491/epics/4/epics
      • api/v4/groups/491/related_epic_links
      • api/v4/groups/491/epics/1/related_epics
    • Observe that the labels array only loads Legacy Epic labels
  • On this MR's branch:
    • Access following endpoints on master branch and then on this branch and look for labels attribute:
      • api/v4/groups/491/epics/
      • api/v4/groups/491/epics/4/epics
      • api/v4/groups/491/related_epic_links
      • api/v4/groups/491/epics/1/related_epics
    • Observe that the labels array loads all labels