Add Planning Hierarchy Page

What does this MR do and why?

Re-introduces the revert MR !76720 (merged) for adding Planning Hierarchy as an EE-only feature.

Also resolves #350471 (closed)

Adds Hierarchy to the Group and Project information, and displays existing planning hierarchy. Add a Current structure and Unavailable structure section to this new Hierarchy page.

Issue: #340844 (closed)

Screenshots or screen recordings

tiers

free tier	premium tier	ultimate tier

How to set up and validate locally

1. Enable the feature flag Feature.enable :work_items_hierarchy
2. Navigate to a project or group information > hierarchy to view the page.

MR acceptance checklist

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