CBOR tag for digests in measurement-values-map

[nedmsmith]

PR #107, line 1555, suggests the need for a CBOR tag for digests measurement, but doesn't include motivation / rationale.

The current documented semantics are that it is an array of digest over the same object. The motivation is algorithm agility.

A different use case might seek to encode digests of different objects. There are multiple options for defining a measurement with different semantics that digests. E.g.: define a new code point, define a different environment-map, create a profile that defines a new code point, define a CBOR tagged digests that encodes semantics in the CBOR tag. Each approach has trade-offs and may have verifier behavior that should be documented.

This issue is intended to capture the trade-offs discussion.