Implement the CoRIM verifier

[thomas-fossati]

Section 8 of the CoRIM I-D describes the expected behaviour of a CoRIM verifier.

Note that only phases 1-4 are in scope and required by compliant implementations, the rest are just informative.

This issue tracks the implementation of phases 1 to 4 of the CoRIM processor.

The expectation is that fully-fledged verifiers (e.g., veraison/services) can use it as a building block for implementing the common "CoRIM-processor" bit, on top of which they can add the rest of the appraisal phases (e.g., policy evaluation, attestation result assembly, and any other deployment and/or product-specific feature).