code: private function buildExtras(ItemInterface $item, array $options)
      {
          if (!empty($options['extras'])) {
              foreach ($options['extras'] as $key => $value) {
                  $item->setExtra($key, $value);
              }
          }
      }
docs: Configures the newly created item's extras
  Extras are processed one by one in order not to reset values set by other extensions
  
  @param ItemInterface $item
  @param array         $options