Improve JSON dumping in CompiledStanModel.with_data #204
Description
Stan's JSON data type has a few custom features, particularly when it comes to complex numbers and tuple types, that json.dumps does not handle correctly.
The JSON processing we use in cmdstanpy is available in the stanio package which is small and standalone. If it was added as a dependency in the [stan] group, then stanio.dump_stan_json would be a great replacement to json.dumps to handle these types (and match e.g. cmdstanpy's behavior in other cases like pandas sequences)