Sessionless evidence submission

[artoniemi]

It should be possible to submit evidence for verification without having to open a session first, i.e. without challenge-response. The verifier-provided challenge/nonce is not needed for freshness in all scenarios. In such cases, having to open a session first creates unnecessary network traffic and round trips. For example, the challenge included in attestation evidence may be channel bindings for a secure channel where the attester and the relying party are the endpoints. In this case, verifying freshness is the task of the relying party instead of the verifier.