Refactor the `Code` class to have separate classes for remote and local codes

[sphuber]

This will simplify the user interface significantly and make it more intuitive. But it will also make the code more maintainable. Ideally, the CLI to create instances of either version should be generated dynamically. This will cater to the future implementation of yet other subclasses of Code. One concrete example would be the ContainerizedCode proposed in #5507