Support optional doc id and auto-uuid generation for non-legacy doctypes

[whd]

We're already planning on supporting this for some legacy systems. We might want to also support it for newer systems since it would probably be low effort to support, and at least one person whose opinion on data is well-respected thought that it was in fact optional.

This would be an update to the edge spec and possibly downstream systems, depending on where the document ID is assigned. It would make for non-idempotent reprocessing of such systems, but that may be acceptable.

[relud]

+1, let's make this issue for actually doing it

[jklukas]

Related: https://github.com/mozilla/gcp-ingestion/issues/668