Register App Attest key

Registers an App Attest key for the current user. The attestation must be made with clientDataHash set to the SHA-256 of the UTF-8 challenge string from a challenge with purpose attest. The server keeps the key id, public key, environment, App ID and counter for at most one hour, or until the next age range submit, and does not keep the attestation itself. Registering the same key again for the same user succeeds and changes nothing. Returns APP_ATTEST_NOT_AVAILABLE with status 404 when this instance does not accept App Attest. Returns FEATURE_NOT_AVAILABLE_SELF_HOSTED with status 403 on a self-hosted instance, and AGE_RANGE_NOT_NEEDED with status 403 when the request does not come from a region where a date of birth typed in is not enough for mature content.