Add new error type for propagating errors from the RoT `Attest` task.

[flihp]

This issue is intended to track the various bits that need to land in order to accomplish: oxidecomputer/hubris#1422. Adding support to sprot for these new commands requires we're able to propagate errors up from the RoT to the SP. This requires converting from SprotError and the new variant we add for the AttestError type from attest-api to the SpError. To do so we need to similarly add new variant to SpError and a new type to mirror the AttestError type.