Support decoding both evidence types

[nick-mobilecoin]

At

mobilecoin/attest/ake/src/responder.rs

Line 145 in 8b2ade2

let remote_report = VerificationReport::decode(payload.as_slice())

and similar locations where the EPID verification report is decoded there will need to be logic to handle the EPID verification report (legacy attestation evidence) or the newer DCAP attestation evidence.

One way to achieve this could be to wrap the new DCAP attestation evidence similar to:

#[derive(Clone, prost::Oneof)]
pub enum Evidence {
    #[prost(message, tag = "4")]
    Dcap(DcapEvidence),
}


#[derive(Clone, prost::Message)]
struct EvidenceMessage {
    #[prost(oneof = "Evidence", tags = "4")]
    evidence: Option<Evidence>,
}   

This would allow one to update the decoding code to be:

let decoded = EvidenceMessage::decode(payload.as_slice()).map_err(|_| Error::Deserialization)?;
match decoded.evidence
    Some(evidence) => verify_dcap_evidence(evidence),
    None => original_epid_logic(payload.as_slice())
}

The new evidence message would only contain data if the protobuf message happened to start with one of the tags that the EvidenceMessage knows about.
This requires that the new evidence message does not use a tag that's found in the legacy EPID verification report.

[nick-mobilecoin]

Done in #3602