DEV: Allow additional TopicList preloaded associations (#18891)

This provides a means to allow additional associations to be preloaded when generating a TopicList.

lib/discourse_plugin_registry.rb

@@ -80,6 +80,7 @@ class DiscoursePluginRegistry
   define_filtered_register :group_params
 
   define_filtered_register :topic_thumbnail_sizes
+  define_filtered_register :topic_preloader_associations
 
   define_filtered_register :api_parameter_routes
   define_filtered_register :api_key_scope_mappings

lib/plugin/instance.rb

@@ -1220,6 +1220,10 @@ class Plugin::Instance
     end
   end
 
+  def register_topic_preloader_associations(fields)
+    DiscoursePluginRegistry.register_topic_preloader_association(fields, self)
+  end
+
   private
 
   def validate_directory_column_name(column_name)