Rework RoT `attest` task for RFD 682

[labbott]

RFD 682 describes our approach to measuring HBS. Our opinions/design on attestation have evolved since we first wrote the attest task API . This issue tracks needed changes.

[labbott]

The work here is pretty straightforward but is semi-dependent on what we want to do on the SP side for actually getting the hash. I'm going to push this out to R23