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.

app/serializers/admin_plugin_serializer.rb

@@ -12,7 +12,8 @@ class AdminPluginSerializer < ApplicationSerializer
              :has_settings,
              :is_official,
              :commit_hash,
-             :commit_url
+             :commit_url,
+             :meta_url
 
   def id
     object.directory_name
@@ -78,4 +79,9 @@ class AdminPluginSerializer < ApplicationSerializer
   def commit_url
     object.commit_url
   end
+
+  def meta_url
+    return if object.metadata.meta_topic_id.blank?
+    "https://meta.discourse.org/t/#{object.metadata.meta_topic_id}"
+  end
 end