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/RequestGenerationMessage.php

@@ -3,6 +3,7 @@
 namespace Chill\DocGeneratorBundle\Service\Messenger;
 
 use Chill\DocGeneratorBundle\Entity\DocGeneratorTemplate;
+use Chill\DocStoreBundle\Entity\StoredObject;
 use Chill\MainBundle\Entity\User;
 
 class RequestGenerationMessage
@@ -13,14 +14,22 @@ class RequestGenerationMessage
 
    private int $entityId;
 
-   private string $entityClassName;
+   private int $destinationStoredObjectId;
 
-    public function __construct(User $creator, DocGeneratorTemplate $template, int $entityId, string $entityClassName)
-    {
+   private array $contextGenerationData;
+
+    public function __construct(
+        User $creator,
+        DocGeneratorTemplate $template,
+        int $entityId,
+        StoredObject $destinationStoredObject,
+        array $contextGenerationData
+    ) {
         $this->creatorId = $creator->getId();
         $this->templateId = $template->getId();
         $this->entityId = $entityId;
-        $this->entityClassName = $entityClassName;
+        $this->destinationStoredObjectId = $destinationStoredObject->getId();
+        $this->contextGenerationData = $contextGenerationData;
     }
 
     public function getCreatorId(): int
@@ -28,6 +37,11 @@ class RequestGenerationMessage
         return $this->creatorId;
     }
 
+    public function getDestinationStoredObjectId(): int
+    {
+        return $this->destinationStoredObjectId;
+    }
+
     public function getTemplateId(): int
     {
         return $this->templateId;
@@ -38,8 +52,8 @@ class RequestGenerationMessage
         return $this->entityId;
     }
 
-    public function getEntityClassName(): string
+    public function getContextGenerationData(): array
     {
-        return $this->entityClassName;
+        return $this->contextGenerationData;
     }
 }