Per-snapshot authorization

We don't have very granular authz for which snapshots atelet is allowed to read. Ideally, an atelet can only read snapshots for Actors that are actively scheduled to it. Several ideas have been discussed, such as GCP IAM conditions or Credential Access Boundaries. However, these are GCP-specific features, and it would be better to do something less cloud specific. GCS, S3, and Azure Storage all support variants of signed URLs, for example.

Potential option:
- On Create (for reading golden snapshot), Suspend, and Resume: ateapi requests a signed URL from the appropriate storage provider and passes it to atelet.
- If this is a performance bottleneck, consider caching pre-signed URLs in ateapi memory to speed it up (we probably can't cache ALL of them, TBD).

This can help mitigate the following threats in the [threat model](https://github.com/agent-substrate/substrate/blob/main/docs/threat-model.md):
- "Attacker who can create ActorTemplates can read or write any storage buckets atelet has access to."
- "A compromised node accesses all snapshots for the cluster."