API: Indicate if label is a project label

changelogs/unreleased/56694-mark-group-level-labels-in-label-api-as-such.yml

+---
+title: 'API: Indicate if label is a project label'
+merge_request: 25219
+author: Robert Schilling
+type: added

lib/api/entities.rb

@@ -1031,6 +1031,9 @@ module API
       expose :priority do |label, options|
         label.priority(options[:parent])
       end
+      expose :is_project_label do |label, options|
+        label.is_a?(::ProjectLabel)
+      end
     end
 
     class List < Grape::Entity