Processes feed data.
\Drupal\aggregator\Plugin\Core\Entity\Feed $feed: A feed object representing the resource to be processed. $feed->items contains an array of feed items downloaded and parsed at the parsing stage. See \Drupal\aggregator\Plugin\FetcherInterface::parse() for the basic format of a single item in the $feed->items array. For the exact format refer to the particular parser in use.
public function process(Feed $feed);