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.

tests/test_auth.py

@@ -0,0 +1,10 @@
+from easybill_client.auth import basic_auth_header, bearer_auth_header
+
+
+def test_bearer_auth_header():
+    assert bearer_auth_header("abc123") == {"Authorization": "Bearer abc123"}
+
+
+def test_basic_auth_header():
+    header = basic_auth_header("user@example.com", "apikey")
+    assert header["Authorization"].startswith("Basic ")