Add support for Apple auth provider

[UniverseXXX]

Would you be able to add "Sign In with Apple" auth provider? Details - https://developer.apple.com/sign-in-with-apple/get-started/, https://developer.apple.com/documentation/signinwithapplejs

Thanks.

[aaronpk]

This would actually be pretty challenging due to the way Apple's OAuth implementation works. There are two things that may be dealbreakers for this.

• Apple does not use static client secrets, they need to be regenerated every so often. That means some not insignificant amount of code to add to this project beyond just configuring new endpoints like other OAuth providers
• Apple does not return any meaningful user identifier to the application. Users have the ability to "hide" their email address if they want. The other identifier returned is specific to the application, and not something people would know ahead of time. That means it would be a particular challenge to hardcode a list of allowed users here for example.