SEP 56: Add datetime to metadata fields

Attach a datetime to (relevant) metadata elements.

Example: the proposed ‘confidence’ metadata could track when a document’s confidence status changed from eg ‘middling’ to ‘high’ in this way.

  - "2023-05-01T12:42:45": "high"
  - "2023-04-12T06:37:01": "middling" 

Alternatively this could all be tracked in a per document changelog, but I think that would brittle, the per document changelog should simply include this information rather than being the arbiter of it.