Feature: [docgen] generate documents in an async queue

The documents are now generated in a queue, using symfony messenger. This queue should be configured:

```yaml
# app/config/messenger.yaml
framework:
    messenger:
        # reset services after consuming messages
        # reset_on_message: true

        failure_transport: failed

        transports:
            # https://symfony.com/doc/current/messenger.html#transport-configuration
            async: '%env(MESSENGER_TRANSPORT_DSN)%'
            priority:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
            failed: 'doctrine://default?queue_name=failed'

        routing:
            # ... other messages
            'Chill\DocGeneratorBundle\Service\Messenger\RequestGenerationMessage': priority
```

`StoredObject`s now have additionnal properties:

* status (pending, failure, ready (by default) ), which explain if the document is generated;
* a generationTrialCounter, which is incremented on each generation trial, which prevent each generation more than 5 times;

The generator computation is moved from the `DocGenTemplateController` to a `Generator` (implementing `GeneratorInterface`. 

There are new methods to `Context` which allow to normalize/denormalize context data to/from a messenger's `Message`.

src/Bundle/ChillDocStoreBundle/Controller/StoredObjectApiController.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Chill\DocStoreBundle\Controller;
+
+use Chill\DocStoreBundle\Entity\StoredObject;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
+use Symfony\Component\Routing\Annotation\Route;
+use Symfony\Component\Security\Core\Security;
+
+class StoredObjectApiController
+{
+    private Security $security;
+
+    public function __construct(Security $security)
+    {
+        $this->security = $security;
+    }
+
+    /**
+     * @Route("/api/1.0/doc-store/stored-object/{uuid}/is-ready")
+     */
+    public function isDocumentReady(StoredObject $storedObject): Response
+    {
+        if (!$this->security->isGranted('ROLE_USER')) {
+            throw new AccessDeniedHttpException();
+        }
+
+        return new JsonResponse(
+            [
+                'id' => $storedObject->getId(),
+                'filename' => $storedObject->getFilename(),
+                'status' => $storedObject->getStatus(),
+                'type' => $storedObject->getType(),
+            ]
+        );
+    }
+}