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/AdvancedDataField.md

@@ -0,0 +1,31 @@
+# AdvancedDataField
+
+EN16931 Business Term (BT field) for structured invoice data
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**identifier** | **str** | BT field identifier (e.g. 'BT-10' for Buyer reference, 'BT-84' for Payment account identifier) | 
+**value** | **str** | Field value | 
+
+## Example
+
+```python
+from easybill_generated_sync.models.advanced_data_field import AdvancedDataField
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of AdvancedDataField from a JSON string
+advanced_data_field_instance = AdvancedDataField.from_json(json)
+# print the JSON string representation of the object
+print(AdvancedDataField.to_json())
+
+# convert the object into a dict
+advanced_data_field_dict = advanced_data_field_instance.to_dict()
+# create an instance of AdvancedDataField from a dict
+advanced_data_field_from_dict = AdvancedDataField.from_dict(advanced_data_field_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)
+
+