feat: add interactive prompts to credential set generator

[technosophos]

This is an attempt to make duffle credentials generate easier to use. This prompts the user to enter values for each credential as they go, instead of requiring the user to merely edit the credential file by hand.

To test, run duffle creds gen on your favorite bundle. Use the new --dry-run flag to test the prompt without writing the file.

[itowlson]

How is this intended to interact with programmatic environments? I can't see an option to silently generate a credentialset - is the idea that we document the credentialset file layout and format, and tooling should write directly to that? It might be good to have a --silent or --infer-empty flag or something that preserves the old behaviour... thoughts?

[bacongobbler]

I do agree with @itowlson's comment about somehow bringing back the old look and feel through a feature flag, however I don't think it needs to be implemented for this to go through. Shall we open a new ticket for that?

[technosophos]

I will add a --no-prompt version that behaves like the older version.

[technosophos]

@itowlson I added a --no-prompt/-q flag that restores the previous behavior.

[technosophos]

Rebased

[radu-matei]

I absolutely love this ❤️

LGTM!