Add unique index, backfill internal id for Attestations

This is a follow-up from #566616 (closed). Internal ID support was added to the SupplyChain::Attestations model, but it was unknown at that time whether any records existed in slsa_attestations—it was expected to be few, if any.

To be safe, a uniqueness validation was not added to (project_id, iid), nor was the index made unique. This issue is concerned with backfilling any possible empty iid values, adding the uniqueness validation, and making the index unique.