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/async/docs/Customers.md

@@ -0,0 +1,33 @@
+# Customers
+
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**page** | **int** | The current page | 
+**pages** | **int** | Max possible pages | 
+**limit** | **int** | Items limitation. Max 1000 | 
+**total** | **int** | Total Items | 
+**items** | [**List[Customer]**](Customer.md) |  | [optional] 
+
+## Example
+
+```python
+from easybill_generated_async.models.customers import Customers
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of Customers from a JSON string
+customers_instance = Customers.from_json(json)
+# print the JSON string representation of the object
+print(Customers.to_json())
+
+# convert the object into a dict
+customers_dict = customers_instance.to_dict()
+# create an instance of Customers from a dict
+customers_from_dict = Customers.from_dict(customers_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)
+
+