Proposal: Define processing results #43

If a processing-expression or processing-execution reference (eg: an openEO Process, OGC API Processes, gdal-calc etc.) produced results to be annotated in a STAC Collection/Item, but that those results contained a mixture of file references (easily integrated as Assets) and data literals (eg: a threshold value or a predicted datetime), the processing extension does not offer a method to indicate those data values.

If the values are simply placed in the Collection fields or Item properties, there is a risk to clash with actual fields that have semantic meaning for STAC (eg: predicted datetime will conflict with the datetime of STAC).

Therefore, the should be a way to provide such values. One approach would be to leverage the processing:expression pre-form object. While this works, it makes it not reusable since anyone can come up with their own thing.