Add labels to work item widgets in GraphQL

app/assets/javascripts/graphql_shared/possible_types.json

app/graphql/types/work_items/widget_interface.rb

app/graphql/types/work_items/widgets/labels_type.rb

app/models/concerns/issuable.rb

app/models/work_items/type.rb

app/models/work_items/widgets/labels.rb

doc/api/graphql/reference/index.md

ee/app/models/concerns/ee/issuable.rb

ee/spec/models/concerns/ee/issuable_spec.rb

ee/spec/models/ee/work_items/type_spec.rb

ee/spec/requests/api/graphql/work_item_spec.rb

spec/graphql/types/work_items/widget_interface_spec.rb

spec/graphql/types/work_items/widgets/labels_type_spec.rb

spec/models/work_item_spec.rb

spec/models/work_items/type_spec.rb

spec/models/work_items/widgets/labels_spec.rb

spec/requests/api/graphql/work_item_spec.rb