import_*: Import data from non-API sources

[m-mohr]

This originates from #83: There are probably several processes, which could be useful to import data from "non-API" sources, similarly to load_uploaded_files. Some ideas:

• import_s3: Load data from Amazon S3 (see example in #83)
• import_nfs: Import from a network file storage system attached to the back-end. Example from VITO: load_disk_data
• import_stac: Import from a STAC catalog/API
• import_external_result: Import a openEO job result from another back-end
• and more (GCS, Azure, ...)

Similarly, there could be export_* processes at some point.

[lforesta]

I like the idea. Maybe we could write explicitly in their description that these processes may require a significant (and possibly slow) data transfer. This may not be clear to non-technical users.

[m-mohr]

More or less encapsulates too many different new processes. There are some separate issues for such processes, e.g. #322