Alternative sources of “workflows”

Currently, all “workflows” are fetched from a hard-coded Zenodo project corresponding to the OSSR.

In practice, this should be configurable (and perhaps we should also support more than one source at a time). Ideally, one could imagine being able to specify a Zenodo project by name. Future extensions could include complete different (non-Zenodo) sources.

Credit to @volodymyr and @ghughes1066 for the suggestion.