FIX: Add plugin event to topic list user lookup (#14116)

This can be used to change the list of topic posters. For example,
discourse-solved can use this to move the user who posted the solution
after the original poster.

lib/plugin/instance.rb

@@ -210,6 +210,16 @@ class Plugin::Instance
     TopicView.add_custom_filter(trigger, &block)
   end
 
+  # Allows to add more user IDs to the list of preloaded users. This can be
+  # useful to efficiently change the list of posters or participants.
+  # Example usage:
+  #   register_topic_list_preload_user_ids do |topics, user_ids, topic_list|
+  #     user_ids << Discourse::SYSTEM_USER_ID
+  #   end
+  def register_topic_list_preload_user_ids(&block)
+    TopicList.on_preload_user_ids(&block)
+  end
+
   # Allow to eager load additional tables in Search. Useful to avoid N+1 performance problems.
   # Example usage:
   #   register_search_topic_eager_load do |opts|