Provider identifier (e.g., 'rekor', 'worm-snapshot').
Whether the external publishing succeeded.
Optional externalExternal reference string (CID, UUID, tx hash, URL, etc.).
Optional metadataProvider-specific metadata (e.g., block number, log index).
Optional errorError message if success is false.
Optional publishedISO 8601 timestamp of when the external publish completed.
Result returned by an AnchorProvider after external publishing.