feat(identifiers): add optional name/description and search to identifiers

Lets identifiers be tagged with a human-readable name and description at creation time, alongside the existing timestamp-based identifier, so experimenters can find a specific run again later without needing to remember its exact identifier string. Adds search_ids, list_detailed_latest_ids, and detailed_latest_id for looking identifiers up by name, plus matching run_id convenience wrappers. Fully backward compatible, all new fields and parameters are optional and default to today's existing behavior.

Closes #243 (closed)