ITE-10: support multiple predicates per step?

[adityasaky]

The current draft for ITE-10 supports multiple predicate types per step, but it's possible this introduces more complexity than is needed. The original idea was to support witness's attestation collections-like workflows, but we may be better off specially handling those types or finding some other mechanism for such steps.

[alanssitis]

IMO, this scenario seems more suitable for something like sub-policies. Although we could do something specific for the collections and handle them differently.

[adityasaky]

Yeah, I'm leaning towards specialcasing collections. There's a PoC of this on attestation-verifier: in-toto/attestation-verifier#26