Work items - Make hierarchy widget type agnostic

What does this MR do and why?

This is a refactor of the hierarchy widget for work items.

As it's currently very tied to OKRs, we are making it type agnostic in order to reuse it for other work item types.

This widget is still only available for Objective work items at this stage.

Screenshots or screen recordings

No user facing changes.

How to set up and validate locally

1. Visit an Objective
2. In Child objectives and key results click on Add dropdown
3. Test the different options and add/create children

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.

Related to #423981 (closed)