Support HTTPS links as redirect URLs on iOS 17.4+

[KhaleelSH]

Supersedes #938, rebased onto 3.0.0 and updated for @w-goog's review there.

For iOS 17.4+, when the redirect URL is an HTTPS universal link, use ASWebAuthenticationSession's HTTPS callback instead of a custom scheme callback.

[w-goog]

The README says "Both Custom URI Schemes and Universal Links can be used with the library.". That hasn't been true for a while - thank you for fixing it!

Also, please add "iOS 17.4+, requires the Associated Domains entitlement and an AASA file for the host" to that line of the README :)

[KhaleelSH]

Thanks for the review! All addressed.

I added webcredentials to your wording, since the HTTPS callback needs the host under that service and apps with only applinks: won't work.

Also one behavior change to flag: the shared handler checks the result of resumeExternalUserAgentFlowWithURL:error: and fails the flow when it returns NO. ASWebAuthenticationSession matches callbacks more loosely than the flow does (a custom scheme matches on the scheme alone, and HTTPS ignores case and the port), so a mismatched redirect used to leave the flow hanging. It now fails with OIDErrorCodeURLMismatch, on the custom scheme path too. Happy to limit that to HTTPS if you'd prefer.

[w-goog]

Looks good! I updated the language around universal links, since they are technically just HTTPS URLs.