Fix N+1 queries in work items REST API

What does this MR do and why?

Fix N+1 queries in work items REST API

• Pass resource_parent option to labels entity to check scoped_labels license once per request instead of per work item
• Add context-aware preloads for project vs group work items
• Preload namespace.route for URL/reference generation
• Preload milestone.project/group for milestone web_url
• Add shared N+1 query prevention test

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable work_item_rest_api feature flag
2. Compare the amount of queries for the request between master and this branch: http://gdk.test:3000/api/v4/namespaces/gitlab/-/work_items?features=milestone,labels&fields=promoted_to_epic_url,web_url,web_path,reference,create_note_email,work_item_type&per_page=100
3. You can compare it by copying the correlation-id from the request header, and add it to the performance bar

MR acceptance checklist

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