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/ChillDocGeneratorBundle/Service/Messenger/RequestGenerationHandler.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace Chill\DocGeneratorBundle\Service\Messenger;
+
+use Chill\DocGeneratorBundle\Repository\DocGeneratorTemplateRepository;
+use Chill\DocGeneratorBundle\Service\Generator\Generator;
+use Chill\DocStoreBundle\Entity\StoredObject;
+use Chill\DocStoreBundle\Repository\StoredObjectRepository;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Component\Messenger\Exception\UnrecoverableMessageHandlingException;
+use Symfony\Component\Messenger\Handler\MessageHandlerInterface;
+
+/**
+ * Handle the request of document generation
+ */
+class RequestGenerationHandler implements MessageHandlerInterface
+{
+    private StoredObjectRepository $storedObjectRepository;
+
+    private DocGeneratorTemplateRepository $docGeneratorTemplateRepository;
+
+    private EntityManagerInterface $entityManager;
+
+    private Generator $generator;
+
+    public const AUTHORIZED_TRIALS = 5;
+
+    public function __construct(
+        DocGeneratorTemplateRepository $docGeneratorTemplateRepository,
+        EntityManagerInterface $entityManager,
+        Generator $generator,
+        StoredObjectRepository $storedObjectRepository,
+    ) {
+        $this->docGeneratorTemplateRepository = $docGeneratorTemplateRepository;
+        $this->entityManager = $entityManager;
+        $this->generator = $generator;
+        $this->storedObjectRepository = $storedObjectRepository;
+    }
+
+    public function __invoke(RequestGenerationMessage $message)
+    {
+        if (null === $template = $this->docGeneratorTemplateRepository->find($message->getTemplateId())) {
+            throw new \RuntimeException('template not found: ' . $message->getTemplateId());
+        }
+
+        if (null === $destinationStoredObject = $this->storedObjectRepository->find($message->getDestinationStoredObjectId())) {
+            throw new \RuntimeException('destination stored object not found : ' . $message->getDestinationStoredObjectId());
+        }
+
+        if ($destinationStoredObject->getGenerationTrialsCounter() >= self::AUTHORIZED_TRIALS) {
+            throw new UnrecoverableMessageHandlingException('maximum number of retry reached');
+        }
+
+        $destinationStoredObject->addGenerationTrial();
+        $this->entityManager->createQuery('UPDATE '.StoredObject::class.' s SET s.generationTrialsCounter = s.generationTrialsCounter + 1 WHERE s.id = :id')
+            ->setParameter('id', $destinationStoredObject->getId())
+            ->execute();
+
+        $this->generator->generateDocFromTemplate(
+            $template,
+            $message->getEntityId(),
+            $message->getContextGenerationData(),
+            $destinationStoredObject
+        );
+    }
+}