Reduce number of spawns in VC attestation service #8520
Description
Description
While reviewing #8171, Jimmy noticed that the attestation service uses spawn a lot, which creates a requirement for lots of error handling. These spawns are somewhat unnecessary when tasks don't need to run in parallel (fetching the attestation data), and we could probably get away with one spawn at the top-level, and making the main attestation service function async.
Even if we do want parallel execution (e.g. for aggregate production), we can accomplish this using join_all.