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.

.gitignore

@@ -0,0 +1,8 @@
+.venv/
+__pycache__/
+.pytest_cache/
+.coverage
+*.pyc
+build/
+dist/
+*.egg-info/