Create App Attest challenge

Creates a single use challenge for the iOS app, valid for 5 minutes. Use purpose attest before registering a new App Attest key, and purpose assert before submitting an age range. 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.