use COSE_CertHash-style digest description in lieu of hash-entry

[thomas-fossati]

We currently import hash-entry from CoSWID:

hash-entry = [
  hash-alg-id: int
  hash-value: bytes
]

which forces hash algorithm identifiers to be integer codes from IANA's named-information registry.

That's great, but imposes a dependency on yet another registry.

We could take instead the more merciful approach taken by RFC9360:

COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ]

which allows, for alg-ids, strings as an alternative to code points.

[nedmsmith]

I'm not passionate about it either way.