Figure out how to expose trust paths for registration verification

[Firehed]

To support §7.1¶23-24

[Firehed]

Since (newly-created) credentials now store their attestation data, explicitly for the purposes of re-verification, I think this may look like adding a method on CredentialInterface that (re-)verifies the attestation object and exposes roughly what's used in the initial registration process.

Older credentials that do not store that data would need to return a "no trust" path; in practice that path shouldn't really exist since they're a pre-1.0 format.