Hierarchy widget - Search and add existing task as work item child

• Expand component created in #362852 (closed) to search existing tasks and add one as child

NOTE: If the Task has an existing parent, that parent should removed when a Task is added as a child of this Issue. We will not support multiple parents at this time.

Design epic of the component for reference: &7063 (closed)

More UX details: #363452 (closed)

Implementation thoughts

Refer to the spike MR for exploratory work: !87191 (closed)

• The search field should be similar to GlFormCombox from gitlab-ui.
• It needs to display more information in the dropdown items (work item ID) and have extra option at the bottom to create a new task.
• It may need to support Object as a value.