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/translations/messages.fr.yml

@@ -10,6 +10,16 @@ docgen:
     test generate: Tester la génération
     With context %name%: 'Avec le contexte "%name%"'
 
+    Doc generation failed: La génération de ce document a échoué
+    Doc generation is pending: La génération de ce document est en cours
+    Come back later: Revenir plus tard
+
+    failure_email:
+        The generation of a document failed: La génération d'un document a échoué
+        The generation of the document {template_name} failed: La génération d'un document à partir du modèle {{ template_name }} a échoué.
+        The following errors were encoutered: Les erreurs suivantes ont été rencontrées
+        Forward this email to your administrator for solving: Faites suivre ce message vers votre administrateur pour la résolution du problème.
+        References: Références
 
 crud:
     docgen_template:
@@ -19,4 +29,4 @@ crud:
 
 
 Show data instead of generating: Montrer les données au lieu de générer le document
-Template file: Fichier modèle
+Template file: Fichier modèle