CBG-4336: add updated at field for persisted configs

[gregns1]

CBG-4336

• Add updated at timestamp to persisted configs
• The following configs are included:
  • Registry
  • Db configs
  • Replication configs
  • User and role docs

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2884/

[bbrks]

@JRascagneres Can you review and make sure the updated_at name and format/UTC aligns with stuff on your side? And input on whether created_at is necessary as well.

[bbrks]

As discussed - add a created_at field as well, but don't care about timestamp format - keep that as-is.