$rendered
$rendered : array|null
The rendered payload, once built.
Base for every event this module dispatches.
Two things every subscriber needs and neither Symfony's Event nor Drupal's supplies: the name the event was dispatched under, so one listener can serve several names, and a JSON body, so a webhook does not have to know the concrete class to render a payload.
The payload is built once and cached, because a webhook dispatcher renders it for every matching subscription and a site can have several.