BE - Work Items: Allow changing parent while reordering children in Hierarchy widget

Summary

Hierarchy Widget supports reordering of children via #370401 (closed) & #385887 (closed) but it is currently limited to reordering amongst siblings, i.e. user cannot move an item into (or out of) an existing work item during drag and drop operation. This has been traditionally possible in legacy Epics as stated in https://docs.gitlab.com/ee/user/group/epics/manage_epics.html#move-child-epics-between-epics, we need to bring back this feature to Hierarchy widget on Work Items.

Legacy experience	Current work item experience

Common use cases

1. Project Restructuring: When a team realizes that a work item fits better under a different parent, they can easily drag and drop it to the new location without having to navigate away from the hierarchy view.
2. Sprint Planning: During sprint planning sessions, product managers can quickly reorganize work items by moving them between different epics or parent items to adjust priorities and workload.
3. Scope Refinement: As project scope evolves, users can effortlessly move work items to more appropriate parent items, ensuring the project structure accurately reflects the current state of work.
4. Cross-team Collaboration: When multiple teams are working on related items, they can easily restructure the hierarchy to group items under the most relevant parent, improving visibility and coordination.
5. Breaking Down Epics: If an epic becomes too large, users can drag and drop some of its children to create new sub-epics or move them under existing ones, helping to manage complexity.
6. Correcting Misplacements: If a work item was initially placed under the wrong parent, users can quickly correct this by dragging it to the proper location in the hierarchy.