The user account object of the message recipient.
Only applies to the user contact form. For a site contact form category, multiple recipients can be configured. The existence of a $recipient triggers user contact form specific processing in the contact message form controller.
@todo Replace Category::$recipients with the user account's e-mail address upon Entity::create().
Type: Drupal\user\Plugin\Core\Entity\User
Drupal\contact\MessageFormController::form()
Drupal\contact\MessageFormController::save()
public $recipient;