Add unit tests for authentication and webhook parsing

- Implement tests for basic and bearer authentication headers in `test_auth.py`.
- Create tests for the `EasybillWebhookParser` in `test_webhooks.py`, covering JSON and form-encoded payloads, as well as a generic parse and acknowledgement method.

generated/sync/docs/Attachment.md

@@ -0,0 +1,36 @@
+# Attachment
+
+If customer_id, project_id and document_id are null, the attachment has a global context and is accessible from the web ui. Keep in mind only to provide one of the four context. You can't attach a file to several context in one request. A error is thrown if you provide two or more context (i. E. sending customer_id, document_id and project_id in combination).
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**created_at** | **date** |  | [optional] [readonly] 
+**customer_id** | **int** |  | [optional] 
+**document_id** | **int** |  | [optional] 
+**file_name** | **str** |  | [optional] [readonly] 
+**id** | **int** |  | [optional] [readonly] 
+**project_id** | **int** |  | [optional] 
+**size** | **int** | In byte | [optional] [readonly] 
+
+## Example
+
+```python
+from easybill_generated_sync.models.attachment import Attachment
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of Attachment from a JSON string
+attachment_instance = Attachment.from_json(json)
+# print the JSON string representation of the object
+print(Attachment.to_json())
+
+# convert the object into a dict
+attachment_dict = attachment_instance.to_dict()
+# create an instance of Attachment from a dict
+attachment_from_dict = Attachment.from_dict(attachment_dict)
+```
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+