Sign in with Apple Unity Plugin v0.4.0

• Support to stop listening to credential revoked notifications by setting the callback to null
• Implemented a nicer example of the plugin, and rearranged the code
• Updated logic for the whole flow
• More documentation in the Readme

Sign in with Apple Unity Plugin v0.3.0

• Rearranged files to support for Unity Package Manager
• Made code more compatible with earlier C# versions

Sign in with Apple Unity Plugin v0.2

• Some more documentation was added
• Added support to listen to Revoked Credentials notifications
• Solved possible crashes that could happen when trying to execute a callback in the Native Message Handler, if the callback was to throw an exception, the application would crash.