STAC clarifications

[m-mohr]

• Allow all STAC versions in range 0.9.x and 1.x.x for being more future proof. 0.9.0 and 1.x.x are vastly compatible and 1.x.x is not allowed to include braking changes anyway (SemVer followed by STAC).
• Beforehand STAC was not clear how to specify unlocated Items, clarified that now that is has been clarified in STAC: Unlocated results set geometry to null and omit the bbox property (applies only to GET /jobs/{job_id}/results).
• Formally forbid 5 elements in bounding boxes.
• Clarify difference between STAC specification and STAC API.
• Clarify that a copy of the STAC Item should be part of the assets in a batch job download.

This is backward-compatible with all back-ends, but may need some subtle changes in clients.

Related to:

• Don't allow 5 elements in bbox radiantearth/stac-spec#844
• bbox definition not strict enough opengeospatial/ogcapi-features#401
• Empty Geometries? radiantearth/stac-spec#843
• Questions related to STAC versions radiantearth/stac-spec#850

[soxofaan]

looks fine (apart from very minor notes)

[m-mohr]

Will merge on Wednesday, if no other comments come up.

[m-mohr]

Due to other demands, merging now. Got feedback from @flahn and @soxofaan and most changes (i.e. Item assets) are not relevant for the Hub or will be "fixed" through js-commons or vue-components, so should not be a problem.