Add support for Apple auth provider

[UniverseXXX]

Add support for Apple auth provider

[Luzifer]

From my first look at this I'm not sure this is possible with how nginx-sso is built and on the other hand whether I want this to be implemented:

• There are interface guidelines how the button has to look; therefore specific buttons are required for the Apple-sign-in
• The suggested method of implementing this is to add a script from the Apple servers - I will not add a third-party script to all sign-in pages. If there is no way around this, there is no chance this gets implemented.

Seeing those points, having seen a quite complex documentation and having no use-case for the Apple provider I'm not going to implement this myself but I'm leaving this open in case someone wants to implement this and contribute a PR.

The only point for the PR is: The implementation may not load a script from Apple servers into the UI if the Apple provider is not currently the active login method.

[Luzifer]

After I just read the reasoning of AnyList not to support "Sign In with Apple" I had a further read for the stuff behind it and decided to go into the same direction: "Sign In with Apple" will not be a supported provider in the nginx-sso core.

Though it's still possible to utilize the plugin system - which I will definitely give some love in near future - to add the provider as a 3rd-party plugin.

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[…] Apple reserves the right to disable Sign in with Apple on a website or app for any reason at any time. […]
Usage Guidelines for Websites and Other Platforms

[…] Apple may receive anonymous and aggregate information about Sign in with Apple, including information about the use of Sign in with Apple. None of the collected information identifies you personally. Apple may use information that does not identify you to improve Apple products and services, for marketing, and for fraud prevention and security purposes. Apple will not attempt to re-identify any anonymous or aggregated Sign in with Apple data we receive. […]
Sign in with Apple & Privacy

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Zero-day in Sign in with Apple