3.1.0

Update API version, add payment models & tests

- Bump version to 3.1.0
- Add new online payment models and types
- Introduce `Payment` class for handling transactions
- Replace `ts-node` with `vitest` for testing
- Remove unused hashing functions (sha1, md5)
- Refactor query parameter handling in services
- Add paginated response type for direct billing service
- Implement unit tests using `vitest`
- Delete old test scripts