load CWL generated STAC collection via load_stac

[jdries]

Users of application packages want to integrate them into larger openEO process graphs, using predefined processes.

I assume the application package generates STAC metadata, as a static catalog in an object storage bucket that is accessible for openEO.

Let's try to pass this as a url directly into load_stac, the user will have to add this 'load_stac' node explicitly for the time being, avoiding to add implicit behaviour.