DEV: Add meta_topic_id plugin metadata (#23838)

For the admin plugin list we want to be able to link to
a meta topic for plugins, but we have no standard way to
do this at the moment. This adds support for meta_topic_id
alongside other plugin metadata like authors, URL etc,
that gets built into a Meta topic URL in the serializer.

plugins/chat/plugin.rb

@@ -6,6 +6,7 @@
 # authors: Kane York, Mark VanLandingham, Martin Brennan, Joffrey Jaffeux
 # url: https://github.com/discourse/discourse/tree/main/plugins/chat
 # transpile_js: true
+# meta_topic_id: 230881
 
 enabled_site_setting :chat_enabled