support universal link as redirect url

[shidoro]

For iOS, when version 17.4+ is available, check whether the redirect url is a universal link. If it is, use the new init method with callback accepting the universal link.

[shidoro]

I believe this is related to #847 and #367

[weihenglim]

Heads up: If you’re planning to use this, note that HTTPS callbacks require the webcredentials service. You’ll need to add it to your AASA and Runner.entitlements if you previously only had applinks. With that in place, I can confirm this PR works great.

[yasmin-ravenwolfe]

I can also confirm - this works great!

Forked AppAuth-iOS locally with this PR applied, with the webcredentials AASA and entitlement config for the redirect url. The OAuth flow completes end-to-end with session-binding routing the callback back to the originating app. Echoing @weihenglim — works as expected with the webcredentials prerequisite documented in their comment.

Is there a maintainer who could pick this up for review? @w-goog — would love your eyes on this when you have a moment. Happy to help shepherd it through / make any requested changes if that helps unblock.