Switch auto-ids to Flake IDs from random UUIDs

This PR starts from @GaelTadh's original PR (#6004) and just folds in the last round of feedback ... I think it's ready?
Because people are not meant to depend on the underlying implementation, what should make someone choose `randomBase64UUID` versus `base64UUID`? I feel like the answer should be documented somewhere (potentially as JavaDoc since it's a utility).