Merge branch '6.4' into 7.0

* 6.4: (24 commits)
  Fix Twig tests
  [Scheduler] Fix tests
  [TwigBridge] Add FormLayoutTestCase class
  Fix CS
  Allow sending scheduled messages through the slack API
  [TwigBridge] Add `AppVariable::getEnabledLocales()` to retrieve the enabled locales
  [RateLimiter] Add missing dependency
  [RateLimiter] Add SlidingWindowLimiter::reserve()
  [Messenger] Add WrappedExceptionsInterface for nested exceptions
  [Mime] Add `TemplatedEmail::locale()` to set the locale for the email rendering
  Fix CS
  [Messenger][Scheduler] Add AsCronTask & AsPeriodicTask attributes
  [Scheduler] Make debug:scheduler output more useful
  [Notifier] Tweak some phpdocs
  [FrameworkBundle] Change BrowserKitAssertionsTrait::getClient() to be protected
  Fix CS
  [FrameworkBundle] Allow BrowserKit relative URL redirect assert
  [Messenger] RejectRedeliveredMessageException should not be retried
  [Serializer] Make `ProblemNormalizer` give details about Messenger’s `ValidationFailedException`
  [WebProfilerBundle] Support `!` negation operator in url filter
  ...

SlackOptions.php

@@ -87,6 +87,16 @@ public function asUser(bool $bool): static
         return $this;
     }
 
+    /**
+     * @return $this
+     */
+    public function postAt(\DateTime $timestamp): static
+    {
+        $this->options['post_at'] = $timestamp->getTimestamp();
+
+        return $this;
+    }
+
     /**
      * @return $this
      */

SlackTransport.php

@@ -76,6 +76,10 @@ protected function doSend(MessageInterface $message): SlackSentMessage
         $options['text'] = $message->getSubject();
 
         $apiMethod = $message->getOptions() instanceof UpdateMessageSlackOptions ? 'chat.update' : 'chat.postMessage';
+        if (\array_key_exists('post_at', $options)) {
+            $apiMethod = 'chat.scheduleMessage';
+        }
+
         $response = $this->client->request('POST', 'https://'.$this->getEndpoint().'/api/'.$apiMethod, [
             'json' => array_filter($options),
             'auth_bearer' => $this->accessToken,