[PQ] SHA-256 usage #2471

CollectedClientData and rpId are currently hashed using SHA-256. This is static with no agility.

SHA-256 is not approved for general hashing use in CNSA 2.0.

Potential options:

1. Request a formal exception in CNSA 2.X for WebAuthn's use of SHA-256 for clientData and rpId
2. @ve7jtb is proposing a signed extension that can carry one or more hashes (SHA-384, SHA-512, etc)
3. Others?