Allow changing child items batch size via query param

Kushal Pandyarequested to merge
479637-update-hierarchy-widget-batch-size into master

What does this MR do and why?

⚠ Important ⚠

This MR is only for testing and is to be reverted once test is complete! See #479637 (comment 2052503291) for more information.

Makes page size for child items dynamic by using query param when provided, while falling back to default when not provided.

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.

How to set up and validate locally

  1. Make sure new epic experience is enabled on your test group, see https://docs.gitlab.com/ee/user/group/epics/epic_work_items.html
  2. Check if you have more than a few child epics present in a given epic.
  3. By default, only 20 children are loaded at a time with Load more button showing at the bottom of the list.
  4. If children_count=50 is provided as a param in the URL, it will use 50 as page size instead of 20.
  5. Once we're able to gather performance data from GitLab.com, we will revert this MR and change value of DEFAULT_PAGE_SIZE_CHILD_ITEMS to a number that fits our requirement.

Related to #479637 (closed)

Edited by Kushal Pandya

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