CLI Checkout: add a `--clean` flag (and an `openfn project clean` command)

[josephjclark]

Sometimes when checking out a new project, it's desirable to preserve as many local and tmp files as possible.

But sometimes you just want to nuke workflows/ from orbit and rebuild it.

--clean and project clean should do exactly the same thing: which means delete the workflows folder and checkout.

Should be easy to add and would be a great fallback for bugs where the CLI fails to track changes properly

[josephjclark]

Mtuchi requests that --clean ignore certain files, like a readme.md.

tricky. It complicates the clean logic a lot, making it more likely to bug. I can't just rm -rf now, I have to loop over files and decide what to do.

Maybe there's a library that would help.

The way I'd handle this is a some configuration in workflow.yaml which includes ignore paths for the clean. But that's probably hard for users to configure too.