validateClientAttestation
Ensures Oauth client attestation attached to the given HTTP request is valid.
See https://drafts.oauth.net/draft-ietf-oauth-attestation-based-client-auth/draft-ietf-oauth-attestation-based-client-auth.html
Return
attestation public key
Throws
request is syntactically incorrect
attestation or attestation proof-of-possession signature is not valid