Allow adding/removing labels from Epics via GraphQL
Proposal
Currently labels can only be added via their ID when using the Epic GraphQL mutations (see https://docs.gitlab.com/ee/api/graphql/reference/#mutationupdateepic and https://docs.gitlab.com/ee/api/graphql/reference/#mutationcreateepic).
Since the GraphQL endpoint uses the same Epics::UpdateService
and Epics::CreateService.new
services there is no technical reason that they cannot support an array of label strings either.