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