This implementation overrides some of PSA's internal backend
functions to handle `state` value with redis as the standard
way doesn't work because of apple sending required details
in the form of POST request.
Includes a mixin test class that'll be useful for testing
Native auth flow.
Thanks to Mateusz Mandera for the idea of using redis and
other important work on this.
Documentation rewritten by tabbott.
Co-authored-by: Mateusz Mandera <mateusz.mandera@zulip.com>