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/easybill_generated_async/api_response.py

@@ -0,0 +1,21 @@
+"""API response object."""
+
+from __future__ import annotations
+from typing import Optional, Generic, Mapping, TypeVar
+from pydantic import Field, StrictInt, StrictBytes, BaseModel
+
+T = TypeVar("T")
+
+class ApiResponse(BaseModel, Generic[T]):
+    """
+    API response object
+    """
+
+    status_code: StrictInt = Field(description="HTTP status code")
+    headers: Optional[Mapping[str, str]] = Field(None, description="HTTP headers")
+    data: T = Field(description="Deserialized data given the data type")
+    raw_data: StrictBytes = Field(description="Raw data (HTTP response body)")
+
+    model_config = {
+        "arbitrary_types_allowed": True
+    }