Near future: add DIDs as suggested identifiers

With many DID variants just months out from production testing, I'd like to submit this in advance to discuss inclusion of identifiers people actually own, DIDs, instead of relatively expensive, insecure, complex identifiers handed out (and revocable by) centralized institutions. I can post the relevant technical integration material as it becomes available.

The issue names no files, tests, or entry points. Start by reviewing the proposed DID variants and the technical integration material referenced in the issue; the issue does not define the integration scope or what completion would look like.

Written by the indexing model from the issue text.