"deep" download functionality of job results

[soxofaan]

job.get_results().download_files() is the existing way to download job result assets
but lacks some features:

• it only supports collection level assets, not item level assets. While openEO 1.1 requires both to be the same, collection-level assets is sub-optimal (e.g. naming-wise) and is scheduled to be removed in openEO 2.0. Item level assets also eliminate ambiguity about which asset belongs to which item
• download of item STAC metadata
• rewrite hrefs to local paths (to make metadata self-contained)
• download additional linked documents like derived_from documents

[soxofaan]

related to

• Downloading STAC files in download_files #184 which also suggests https://github.com/stac-utils/stac-asset