Add ability to mutate labels within Epic in GraphQL
Summary
Use GraphQL API to mutate Labels in an Epic, for use on the Epic sidebar.
Improvements
- As part of the sidebar refactor, we will use the GraphQL API. This change will add the ability to mutate labels within an Epic, so that they can be added and removed within the sidebar
This issue is blocked by #31912 (closed), which will add the labels attribute to Epics (as it already is in EpicIssue)