Figure out a way to communicate accepted I/O schema version to submitters

Now that we support multiple database schemas, a deployment might not always accept the latest I/O schema. That's something verified only on the server (Cloud Function) side, **after** a submission went through the submission message queue, and by then there's no way to tell the submitter if their schema version is too new for the deployment. Figure out a way to communicate that to submitters before they send their data, have the data validated, and abort on error, if it's too new.

Start by tracing the submission message queue and the server-side Cloud Function validation described in the issue. Determine how a deployment can expose its accepted I/O schema version before submission. Done means submitters learn the supported version before sending data, and incompatible submissions are prevented before validation fails.

Written by the indexing model from the issue text.