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/Resources/views/Email/on_generation_failed_email.txt.twig

@@ -0,0 +1,16 @@
+{{ creator.label }},
+
+{{ 'docgen.failure_email.The generation of the document {template_name} failed'|trans({'{template_name}': template.name|localize_translatable_string}) }}
+
+{{ 'docgen.failure_email.Forward this email to your administrator for solving'|trans }}
+
+{{ 'docgen.failure_email.References'|trans }}:
+{% if errors|length > 0 %}
+{{ 'docgen.failure_email.The following errors were encoutered'|trans }}:
+
+{% for error in errors %}
+- {{ error }}
+{% endfor %}
+{% endif %}
+- template_id: {{ template.id }}
+- stored_object_destination_id: {{ stored_object_id }}