Create Play Integrity challenge

Creates a single use challenge for the Android app, valid for 5 minutes. Put it in client_data before submitting an age range with a Play Integrity token. Returns PLAY_INTEGRITY_NOT_AVAILABLE with status 404 when this instance does not accept Play Integrity. 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.