Add Planning Hierarchy Page

What does this MR do and why?

Re-introduces the revert MR !76720 (merged) for adding Planning Hierarchy as-is.

Confusion was caused by adding GitLab Premium label to the original issue while this is for GitLab FOSS too.

Also resolves #350471 (closed) (move controller specs to request spec)

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 Information > Planning Hierarchy to view the page.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.