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/config/services.yaml

@@ -20,10 +20,14 @@ services:
         resource: '../Serializer/Normalizer/'
         tags:
             - { name: 'serializer.normalizer', priority: -152 }
+
     Chill\DocGeneratorBundle\Serializer\Normalizer\CollectionDocGenNormalizer:
         tags:
             - { name: 'serializer.normalizer', priority: -126 }
 
+    Chill\DocGeneratorBundle\Service\Context\:
+        resource: "../Service/Context"
+
     Chill\DocGeneratorBundle\Controller\:
         resource: "../Controller"
         autowire: true
@@ -34,18 +38,20 @@ services:
         autowire: true
         autoconfigure: true
 
-    Chill\DocGeneratorBundle\Service\Context\:
-        resource: "../Service/Context/"
-        autowire: true
-        autoconfigure: true
-
     Chill\DocGeneratorBundle\GeneratorDriver\:
         resource: "../GeneratorDriver/"
         autowire: true
         autoconfigure: true
 
+    Chill\DocGeneratorBundle\Service\Messenger\:
+        resource: "../Service/Messenger/"
+
+    Chill\DocGeneratorBundle\Service\Generator\Generator: ~
+    Chill\DocGeneratorBundle\Service\Generator\GeneratorInterface: '@Chill\DocGeneratorBundle\Service\Generator\Generator'
+
     Chill\DocGeneratorBundle\Driver\RelatorioDriver: '@Chill\DocGeneratorBundle\Driver\DriverInterface'
 
     Chill\DocGeneratorBundle\Context\ContextManager:
         arguments:
             $contexts: !tagged_iterator { tag: chill_docgen.context, default_index_method: getKey }
+    Chill\DocGeneratorBundle\Context\ContextManagerInterface: '@Chill\DocGeneratorBundle\Context\ContextManager'