Fix per-platform Apple token config

[gabriel]

This pull request refactors the AppleAuth::Token class to make it more flexible and testable by allowing a custom configuration object to be passed in, rather than relying on a global configuration. All internal references to configuration values have been updated to use the instance-specific config object.

Configuration injection and usage:

• The AppleAuth::Token initializer now accepts a config: keyword argument, defaulting to AppleAuth.config, and stores it as an instance variable. All internal references to configuration values (such as apple_client_id, apple_team_id, redirect_uri, apple_key_id, and apple_private_key) now use this instance variable instead of the global config. [1] [2] [3] [4] [5]

Code improvements:

• Improved type checking for the private key by replacing a direct class comparison with is_a?, making the code more idiomatic and robust.### Summary

Other Information