[RPD-306] Add core stack add command

[Christopher-Norman]

This PR adds the matcha stack add <module type> <module flavor> command.

Upon a successful command run the matcha.config.json file will be updated:

 % matcha stack add experiment_tracker mlflow
Matcha 'experiment_tracker' module of flavor 'mlflow' has been added to the current stack.

matcha.config.json file:

{"stack": {"name": "custom", "experiment_tracker": "mlflow"}}

along with the stack name "custom"

Checklist

Please ensure you have done the following:

• [x] I have read the CONTRIBUTING guide.
• [ ] I have updated the documentation if required.
• [x] I have added tests which cover my changes.

Type of change

Tick all those that apply:

• [ ] Bug Fix (non-breaking change, fixing an issue)
• [x] New feature (non-breaking change to add functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Other (add details above)