Adds relative position handling to Work Item Create mutation

What does this MR do and why?

Enables default behavior for work item relative positioning when calling Mutations::WorkItems::Create. When parentId input is given, new child is placed at the bottom of the hierarchy.

Related to #370401 (closed)

Implements #370401 Task Mutations::WorkItems::Create should place the new child at the bottom of the hierarchy

Screenshots or screen recordings

How to set up and validate locally

1. Local Test scenario background setup.

• Create a new Issue named Testing Work Items Hierarchy Widget children positioned creation:
  • Add six tasks to the issue named Task #6, Task #1, Task #2 ... Task #5.
  • Verify new tasks are added to the end of the list, which is consistent behavior.
  • Reload the page in the browser, and verify Task #1 appears at the top of the list.

2. Local Test scenario verification.

• Reload Testing Work Items Hierarchy Widget children positioned creation issue page in the browser and verify:
  • All six tasks are present on the page.
  • Work Item Task #6 appears at the top of the list.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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