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.

examples/webhook_receiver_example.py

@@ -0,0 +1,10 @@
+from easybill_client import EasybillWebhookParser
+
+
+def handle_easybill_webhook(headers: dict[str, str], payload: dict, content_type: str) -> tuple[int, dict]:
+    parser = EasybillWebhookParser()
+    envelope = parser.parse(headers=headers, payload=payload, content_type=content_type)
+
+    print(f"Received easybill event: {envelope.event} for resource {envelope.resource_id}")
+
+    return 200, parser.build_acknowledgement()