Make checkout step optional

[adrogon]

Describe Request:

The attach_workspace step is currently performed after the checkout one.

attach_workspace is useful if a previous job contained a persist_to_workspace step.
If said job already contained a checkout step, the Path Filtering Orb performs a checkout for no reason.

Making the checkout step of this orb optional through configuration would allow for more use cases; for example, retrieving the repository by attaching the workspace, or restoring a cache.

Thank you.

[Fernando-Abreu]

I'm closing this ticket in favour of this one: #68