BE: Add support for setting Work Item as parent to an Issue
Summary
Note: This is part of &9290, where we are planning to migrate Epics to Work Items.
Since Epics to Work Items migration is scheduled to happen before we're able to migrate legacy Issues to Work Items, we need a way to associate a legacy issue as a child to an Epic Work Item, this is only needed until Issues too are Work Items as then, we get parent-child relationships between Work Items for free via Hierarchy Widget that we built for OKRs via &7864.
Edited by Kushal Pandya