Make `AuthenticatorAttestationResponseJSON.publicKeyAlgorithm` optional

[zacknewman]

The motivation behind both AuthenticatorAttestationResponseJSON.publicKey and AuthenticatorAttestationResponseJSON.publicKeyAlgorithm is the same: easy access to credential data. For good reason though, AuthenticatorAttestationResponseJSON.publicKey is not required since technically such data exists in the required AuthenticatorAttestationResponseJSON.attestationObject. I believe the same should be true for AuthenticatorAttestationResponseJSON.publicKeyAlgorithm since it doesn't really serve purpose without AuthenticatorAttestationResponseJSON.publicKey also existing.