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/ChillDocStoreBundle/Resources/public/vuejs/StoredObjectButton/helpers.ts

@@ -1,3 +1,4 @@
+import {StoredObject, StoredObjectStatus, StoredObjectStatusChange} from "../../types";
 
 const MIMES_EDIT = new Set([
   'application/vnd.ms-powerpoint',
@@ -168,6 +169,18 @@ async function download_and_decrypt_doc(urlGenerator: string, keyData: JsonWebKe
    }
 }
 
+async function is_object_ready(storedObject: StoredObject): Promise<StoredObjectStatusChange>
+{
+    const new_status_response = await window
+      .fetch( `/api/1.0/doc-store/stored-object/${storedObject.uuid}/is-ready`);
+
+    if (!new_status_response.ok) {
+      throw new Error("could not fetch the new status");
+    }
+
+    return await new_status_response.json();
+}
+
 export {
   build_convert_link,
   build_download_info_link,
@@ -176,4 +189,5 @@ export {
   download_doc,
   is_extension_editable,
   is_extension_viewable,
+  is_object_ready,
 };