Possible ITE: Resolve attestation references during verification

[adityasaky]

Some ITE-6 attestations like SCAI and VSA can reference other attestations. During verification, it may be necessary to resolve these references and apply rules against the target attestations.

Existing work: @marcelamelara has prototyped some support for this attestation resolving behaviour for SCAI. As a next step, we should generalize this for resolving references in all predicates and formally specify how it works. This may require a new ITE, alongside ITEs 10 and 11 which update the layout schema to support attestations.

This was first raised by @MarkLodato during a review of ITE-11: #50 (comment)

[trishankatdatadog]

Interesting problem, but this may be going down a tricky road unless we have things like PURLs. I think it's up to clients to pass in a complete bundle to verifiers, but would be good to consider other options.

[adityasaky]

Right, one thing we'd have to do is work out the scope for this. Note that in the simplest case, this could be scoped precisely to the bundle passed to a verifier. Even then, if the verification encounters a SCAI predicate, the layout may call for resolving the evidence for some attribute to the full attestation in the same bundle to apply other rules.