Support attestation proof

[vafeini]

Draft 15 intoruces the concept of "key attestation" as a way for the holder to provide evidence for the authenticity and security properties of a key and its storage component to the Credential Issuer.

Spec forsees that a key attestation jwt can be used as proof of possession of key material to be embedded in issued credentials (see here)

It also mentiones that

The Credential Issuer SHOULD issue a Credential for each cryptographic public key specified in the attested_keys claim within the key_attestation parameter.

For the above to be supported issuer must:

• Introduce new proof type attestation in credential configuration's proof_types_supported
• Validate the key attestation passed from wallet:
  • Validate that key attestation is required from the credential's policy (configuration includes it in proof_types_supported)
  • Validate signature of key attestation as described in Support key attestations in JWT proofs #277
  • Validate that key attestation is aligned with the specs in credential configuration (key_attestations_required.key_storage and key_attestations_required.user_authentication )
• For every key included in key attestation's attested_keys claim a separate credential instance should be created for the same dataset and format.