Updated phpdoc for \Drupal\scheduled_updates\Plugin\BaseUpdateRunner::getEntityIdsReferencingReadyUpdates

Added new interface EntityMonitorUpdateRunnerInterface for runners that need to react to updates of entities that type will update them

Issue #2647074 by tedbow: WebTest - update ReferencerScheduledUpdateTypeTest to test adding multiple types

Issue #2647074 by tedbow: WebTest - \Drupal\scheduled_updates\Tests\ReferencerScheduledUpdateTypeTest

Update ScheduledUpdateTypeInterface constant values to avoid use 0. Scheduled Update Types will need to be resaved

Issue #2642522 by tedbow: Create option to create or update entity reference field on target entity type

Fixed \Drupal\scheduled_updates\Plugin\UpdateRunner\DefaultUpdateRunner::getReferencingUpdates because it was getting referenced updates