RefValIDs and TrustAnchorIDs in `proto` Evidence Context

[yogeshbdeshpande]

The current proto interface between Verification Front End and VTS returns an AppraisalContext which has the
EvidenceContext.

It is NOT clear as to following:

1. Why the EvidenceContext should have trust-anchor-ids &reference-ids
2. What is the need of the same to be returned to the Verification front end?
3. Even if it is needed, it is not the correct structure to hold this.