Lead Verifier: RATSd lead attester plugin

[thomas-fossati]

The RATSd lead attester represents each component attester as a child node in a CMW collection. The overall evidence is signed with the lead attester key.

This plugin is only concerned with the high-level shape of the evidence, specifically:

1. Verifying the evidence signature against a configured trust anchor;
2. Ensuring that the assembly conforms to the RATSd API.

It is assumed that the individual composite evidence will be handled by the composite verifier logic.