allow entity types to specify their own bundle_plugin handler class

Is it as simple as just checking whether the entity type has a bundle_plugin handler class, and only if not do we set the default?

See attached patch or below:

function entity_entity_type_build(array &$entity_types) {
  foreach ($entity_types as $entity_type) {
    if ($entity_type->get('bundle_plugin_type')) {
      if (!$entity_type->hasHandlerClass('bundle_plugin')) {
        $entity_type->setHandlerClass('bundle_plugin', BundlePluginHandler::class);
      }
    }
  }
}

No reason not to. Thanks!