Support selecting project when creating child work items via hierarchy widget

The Hierarchy Widget under Work Items supports creating child work items and as of today, it only supports title, we need to add support for selecting a child project within this form.

Current	Proposed

• Create a new group where you want to use to develop this feature, it needs to have EE Ultimate enabled (which is by default as long as EE license on your GDK is working as expected).
• Enable following feature flags on this group, it'll ensure group-level work items are fully enabled on this group;
  • work_items_mvc_2
  • namespace_level_work_items
  • sync_epic_to_work_item
  • sync_epic_work_item_order
• Disable make_synced_work_item_read_only
• Go to https://gdk.test:3000/groups/path/to/group/-/work_items and you should see a Create epic button
• Create epic by setting a title and then open it and you should see the hierarchy widget as shown in screenshot