Integrate Apalache Chai

Currently, for interacting cleanly with Apalache, we use [modelator-py](https://github.com/informalsystems/modelator-py).
However, recently, Apalache introduced the server mode called [Chai](https://github.com/informalsystems/apalache-chai). `Chai` would give us the same functionality as `modelator-py` but would be more robust and standardized (ie, we would not depend on hacky parsing of CLI output).