Support for per-scheme trust anchors

[thomas-fossati]

Problem

At present, Veraison assumes the binding of a trust anchor / verification key to an attesting environment.

Some attestation schemes (e.g., AWS Nitro) rely on a global "per-scheme" trust anchor, which is the same irrespective of the attesting environment that produces the evidence.

It should be possible for trust anchors to be associated with just an attestation scheme.

Possible solution

The natural way to achieve this is by using CoTS.

Additional context

Note that this depends on veraison/corim#64