DEV: Allow plugins to request topic thumbnail sizes (#9828)

In plugin.rb, you can register new sizes like

```
register_topic_thumbnail_size [512, 512]
```

For more information about thumbnails see 03818e642a

lib/discourse_plugin_registry.rb

@@ -75,6 +75,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :editable_group_custom_fields
 
+  define_filtered_register :topic_thumbnail_sizes
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end