Add processing:version and processing:datetime

[m-mohr]

• processing:version to describe the primary software version of workflow version that produced the data
• processing:datetime to describe when the processing happened
• processing-software relation type

This is meant to deprecate fields such as:

• landsat:product_generated
• s2:generation_time
• s2:processing_baseline
• s1:processing_datetime

[m-mohr]

I'm just wondering whether we should just use the version extension, i.e. the version field, for the processing baseline version.

[emmanuelmathot]

That is pretty different for me. version field is for the stac document version whilst processing version is for the processor. There will be cases where the data processing will remain the same across multiple document versioning. E.g. Sentinel ground segment IPF processor is different from the CDSE cataloging processor version.

[m-mohr]

I've updated the PR with more descriptions and explanations.

I've also added a new relation type "processing-software" to cover #29