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/PdfTemplatesApi.md

@@ -0,0 +1,94 @@
+# easybill_generated_async.PdfTemplatesApi
+
+All URIs are relative to *https://api.easybill.de/rest/v1*
+
+Method | HTTP request | Description
+------------- | ------------- | -------------
+[**pdf_templates_get**](PdfTemplatesApi.md#pdf_templates_get) | **GET** /pdf-templates | Fetch PDF Templates list
+
+
+# **pdf_templates_get**
+> PDFTemplates pdf_templates_get(type=type)
+
+Fetch PDF Templates list
+
+### Example
+
+* Basic Authentication (basicAuth):
+* Api Key Authentication (Bearer):
+
+```python
+import easybill_generated_async
+from easybill_generated_async.models.pdf_templates import PDFTemplates
+from easybill_generated_async.rest import ApiException
+from pprint import pprint
+
+# Defining the host is optional and defaults to https://api.easybill.de/rest/v1
+# See configuration.py for a list of all supported configuration parameters.
+configuration = easybill_generated_async.Configuration(
+    host = "https://api.easybill.de/rest/v1"
+)
+
+# The client must configure the authentication and authorization parameters
+# in accordance with the API server security policy.
+# Examples for each auth method are provided below, use the example that
+# satisfies your auth use case.
+
+# Configure HTTP basic authorization: basicAuth
+configuration = easybill_generated_async.Configuration(
+    username = os.environ["USERNAME"],
+    password = os.environ["PASSWORD"]
+)
+
+# Configure API key authorization: Bearer
+configuration.api_key['Bearer'] = os.environ["API_KEY"]
+
+# Uncomment below to setup prefix (e.g. Bearer) for API key, if needed
+# configuration.api_key_prefix['Bearer'] = 'Bearer'
+
+# Enter a context with an instance of the API client
+async with easybill_generated_async.ApiClient(configuration) as api_client:
+    # Create an instance of the API class
+    api_instance = easybill_generated_async.PdfTemplatesApi(api_client)
+    type = ['type_example'] # List[str] | Filters the templates by the specified type. You can specify several types comma-separated, like type,type,type. (optional)
+
+    try:
+        # Fetch PDF Templates list
+        api_response = await api_instance.pdf_templates_get(type=type)
+        print("The response of PdfTemplatesApi->pdf_templates_get:\n")
+        pprint(api_response)
+    except Exception as e:
+        print("Exception when calling PdfTemplatesApi->pdf_templates_get: %s\n" % e)
+```
+
+
+
+### Parameters
+
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **type** | [**List[str]**](str.md)| Filters the templates by the specified type. You can specify several types comma-separated, like type,type,type. | [optional] 
+
+### Return type
+
+[**PDFTemplates**](PDFTemplates.md)
+
+### Authorization
+
+[basicAuth](../README.md#basicAuth), [Bearer](../README.md#Bearer)
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: application/json
+
+### HTTP response details
+
+| Status code | Description | Response headers |
+|-------------|-------------|------------------|
+**200** | Successful operation |  -  |
+**429** | Too Many Requests |  -  |
+
+[[Back to top]](#) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to Model list]](../README.md#documentation-for-models) [[Back to README]](../README.md)
+