Config entities that use plugin bags should implement EntityWithPluginBagInterface

If your ConfigEntity class uses a PluginBag in order to manage its plugins, you now should implement Drupal\Core\Config\Entity\EntityWithPluginBagInterface.
This will provide automatic handling of your plugin's configuration, keeping it in sync when an entity is saved.
If you are storing your plugin's configuration on the entity in a property other than $configuration, you must specify that as follows in your entity class:
protected $pluginConfigKey = 'my_settings';