add AttendanceCertificateEvent symfony event

Adds a Symfony Event broadcast when an attendance certificate is about to be generated. This allows implementers to conditionally choose a message template id in custom extensions. To use: Add an event subscriber in a custom extension. Example, create file in <extension_dir>/Civi/Event/Subscriber named AttendanceEventSubscriber.php

<?php

namespace Civi\Event\Subscriber;

use Civi\Certificates\Event\AttendanceCertificateEvent;
use Civi\Core\Service\AutoSubscriber;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Civi\Api4\Participant;

/**
 * This requires <mixin>scan-classes@1.0.0</mixin> in the extension info.xml in the <mixins> area.
 * @service
 * @internal
 */
class AttendanceEventSubscriber extends AutoSubscriber implements EventSubscriberInterface {

  public static function getSubscribedEvents(): array {
    return [
      'civi.certificates.attendance' => 'determineMessageTemplateID',
    ];
  }

  /**
   * @param \Civi\Certificates\Event\AttendanceCertificateEvent $event
   */
  public function determineMessageTemplateID(AttendanceCertificateEvent $event): void {
    $contact_id = $event->getContactId();
    $participant_id = $event->getParticipantId();
    $event_id = $event->getEventId();
    $message_template_id = $event->getMessageTemplateId();
    $event->setMessageTemplateID(999);
  }

}

Make sure the extension info.xml has the scan-classes mixin specified

  <mixins>
    <mixin>scan-classes@1.0.0</mixin>
  </mixins>