Refactoring debusine client CLI

The CLI of debusine-client keeps growing (which is a good thing). We currently see two issues:

• cli.py getting bigger and bigger
• The debusine command line syntax could benefit with slowly migrating to a more consistent design, such as debusine object verb --options

We currently don't have a clear idea of how to get there: do we stick with argparse? Are there better CLI frameworks out there that we could use? What would be a good refactoring of cli.py?

I'm opening this issue to track design ideas.