Add support for export_workspace

[jdries]

after doing save_result, a user should be able to do export_workspace on the stac metadata.

To be fully correct, we need a new type of object that represents stac items:
#402

[soxofaan]

some more refs and specs

• STAC, Collection and Workspace handling in openEO openeo-processes#485
  • save_results: Returns the STAC resource instead of boolean true -> DataCube.save_result returns DataCube instead of (wrapped) true #402 (comment)
• export_to_workspace openeo-processes#467
• https://github.com/Open-EO/openeo-processes/blob/draft/proposals/export_workspace.json
• release 2.0.0-rc.2 ? openeo-processes#530