Draft: Add pagination support for Hierarchy Widget children list

What does this MR do and why?

Add pagination support for work item's Hierarchy Widget children.

Notes:

1. Current limit is set to 2 for testing purpose. Before merging the MR, it will be set to 50 as per the discussion.
2. Subscription will be resolved separately in the Draft: Fix subscription in hierarchy widget (!158567)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen Recording 2024-07-03 at 10.10.27 PM.mov

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable Epic work items Feature.enable(:namespace_level_work_items)

Steps to verify:

1. Login with any user
2. Go to any Group or Project and create an Epic or Objective
3. Add more than 3 items in Child items section
4. Check if Load more button is appearing
5. Check if reordering is working after loading more items
6. Add few items using the Add button functionality.
7. Try to remove few items from the list

Check similar behaviour on Issue's Child items widget.

Related to #463881 (closed)