Add additional converter methods to FmfContext, StepData and Discover/shell

[therazix]

Adds several to_minimal_spec methods and one to_serialized method. It also makes WhereableStepData a serializable container. This is needed for the recipe loading feature (#4208).

Pull Request Checklist

• [x] implement the feature
• [ ] write the documentation
• [ ] extend the test coverage
• [ ] update the specification
• [ ] adjust plugin docstring
• [ ] modify the json schema
• [ ] mention the version
• [ ] include a release note