DEV: Add true_fields method for CustomFields (#24876)

This is useful for plugins that might otherwise rely on the
CUSTOM_FIELD_TRUE constant.

app/models/topic_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class TopicCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :topic
 end