Add Step for fetching external resources

[pydsigner]

Add a step that uses a config file to pull in an external resource, either from the internet or from the local filesystem.

[pydsigner]

We will have to extend the new explicit custody return values for Step, but only for input sources. This is important for handling resources that come from outside the filesystem, and does not really cost us anything; if we do not support checking whether non-FS resources have changed, they can simply be treated as always dirty. We will ideally want to have an API for custody management, which will allow end users to support their own resource types as needed. Output paths can remain strictly paths, since the flow of external data is strictly ingress in our approach to a file processing engine.