Move eventable types constant to a helper

This moves API::ResourceLabelEventsHelpers::EVENTABLE_TYPES to a class
method in a separate helper module, allowing EE to redefine it without
having to directly modify the source code.

lib/api/helpers/resource_label_events_helpers.rb

+# frozen_string_literal: true
+
+module API
+  module Helpers
+    module ResourceLabelEventsHelpers
+      def self.eventable_types
+        # This is a method instead of a constant, allowing EE to more easily
+        # extend it.
+        [Issue, MergeRequest]
+      end
+    end
+  end
+end

lib/api/resource_label_events.rb

@@ -7,9 +7,7 @@ module API
 
     before { authenticate! }
 
-    EVENTABLE_TYPES = [Issue, MergeRequest].freeze
-
-    EVENTABLE_TYPES.each do |eventable_type|
+    Helpers::ResourceLabelEventsHelpers.eventable_types.each do |eventable_type|
       parent_type = eventable_type.parent_class.to_s.underscore
       eventables_str = eventable_type.to_s.underscore.pluralize