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/Context/DocGeneratorContextWithPublicFormInterface.php

@@ -23,6 +23,9 @@ interface DocGeneratorContextWithPublicFormInterface extends DocGeneratorContext
      */
     public function buildPublicForm(FormBuilderInterface $builder, DocGeneratorTemplate $template, $entity): void;
 
+    /**
+     * Fill the form with initial data
+     */
     public function getFormData(DocGeneratorTemplate $template, $entity): array;
 
     /**
@@ -31,4 +34,14 @@ interface DocGeneratorContextWithPublicFormInterface extends DocGeneratorContext
      * @param mixed $entity
      */
     public function hasPublicForm(DocGeneratorTemplate $template, $entity): bool;
+
+    /**
+     * Transform the data from the form into serializable data, storable into messenger's message
+     */
+    public function contextGenerationDataNormalize(DocGeneratorTemplate $template, $entity, array $data): array;
+
+    /**
+     * Reverse the data from the messenger's message into data usable for doc's generation
+     */
+    public function contextGenerationDataDenormalize(DocGeneratorTemplate $template, $entity, array $data): array;
 }