Issue #3450070: Implement post cache webhook event

Closes #3450070

src/Event/CacheWebhookResponseEvent.php

+<?php
+
+namespace Drupal\nextjs\Event;
+
+use Psr\Http\Message\ResponseInterface;
+use Symfony\Contracts\EventDispatcher\Event;
+
+/**
+ * Event when a cache webhook was sent.
+ */
+class CacheWebhookResponseEvent extends Event {
+
+  const EVENT_NAME = 'nextjs.cache_webhook_response';
+
+  /**
+   * Construct new event.
+   */
+  public function __construct(
+    protected string $url,
+    protected ResponseInterface $response,
+    protected \DateTime $dateTime,
+  ) {}
+
+  /**
+   * If webhook was processed successfully.
+   */
+  public function isSuccess(): bool {
+    return $this->response->getStatusCode() === 200;
+  }
+
+  /**
+   * Get HTTP response.
+   */
+  public function getResponse(): ResponseInterface {
+    return $this->response;
+  }
+
+  /**
+   * Get time when webhook was processed by Next.js.
+   *
+   * This is useful, e.g. if you need to make sure the cache
+   * revalidation is fully propagated (add 300ms to this time).
+   *
+   * @see https://vercel.com/docs/infrastructure/data-cache#revalidation-behavior
+   */
+  public function getProcessedTime(): \DateTime {
+    return $this->dateTime;
+  }
+
+}