title: Symfony messenger and mailer : how to add a binding_key?
body: <p>I have a running Symfony 4.4 project with messenger and rabbitMQ.
  I have an async transport with 2 queues.</p>
  <pre><code>    transports:
          # https://symfony.com/doc/current/messenger.html#transport-configuration
        async:
          dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
          options:
            exchange:
              name: myexchange
              type: direct
            queues:
              email:
                binding_keys:
                  - email
              extranet:
                binding_keys:
                    - extranet
          # failed: 'doctrine://default?queue_name=failed'
          # sync: 'sync://'
  
      routing:
          # Route your messages to the transports
          'App\Message\ExtranetMessage': async
          'Symfony\Component\Mailer\Messenger\SendEmailMessage': async
  </code></pre>
  <p>I need to send email with the <code>symfony/mailer</code> component to the email queue.</p>
  <pre><code>public function contact(Request $request, MailerInterface $mailer)
  {
      if($request-&gt;isXmlHttpRequest())
      {
          //dd($request-&gt;request-&gt;all());
          $body =
              'Nouveau message depuis le front&lt;br /&gt;
               Nom = '.$request-&gt;request-&gt;get('nom').'&lt;br /&gt;
               Prénom = '.$request-&gt;request-&gt;get('prenom').'&lt;br /&gt;
               Société = '.$request-&gt;request-&gt;get('societe').'&lt;br /&gt;
               Email = '.$request-&gt;request-&gt;get('mail').'&lt;br /&gt;';
  
          $email = (new Email())
              -&gt;from('from@email.tld')
              -&gt;replyTo($request-&gt;request-&gt;get('mail'))
              -&gt;to('$request-&gt;request-&gt;get('mail')')
              -&gt;subject('test')
              -&gt;html($body);
  
          $mailer-&gt;send($email);
  
          return new JsonResponse('OK', 200);
      }
  }
  </code></pre>
  <p>How can I add the binding_key to the <code>mailer</code> in order to let rabbitMQ know how to handle the email ?</p>