[EPIC] STAC metadata upgrade & rework

[jdries]

Our own STAC metadata generation is showing its age and is not correct in some places. It's becoming increasingly urgent to rework this.

1. Support newer STAC versions
2. asset per band: group assets in item, do not create one item per assets
3. define item_assets in collection metadata
4. Asset keys should not be item id's or contain dates. They should be the same across the collection.

I propose to make the STAC version of the output a format or job option? We need to offer a proper migration path, also considering that large scale production projects may already depend on the current form of STAC metadata we create.

[jdries]

Design idea: scala side returns a list of preliminary item json objects rather than a list of asset filenames.
The python side then transforms/augments the preliminary json into fully stac compliant json, allowing a fairly generic code path for the STAC metadata construction.