Allow Custom Nonce Value for Apple Provider

[SyedAli00896]

Goals

1. Allow the Apple Provider in next-auth to accept custom nonce values.

Non-Goals

1. This feature request does not aim to change the general flow or security model of the Apple sign-in process.

Background

The current implementation of next-auth's Apple Provider automatically computes a SHA256 hashed nonce value, which is suitable for some use cases. However, some backend SDKs, such as Firebase, only allow the use of rawNonce for verifying the id_token. This creates friction for developers who need to verify the id_token on the backend using the rawNonce.

Proposal

This feature could be implemented by adding an option in the Apple Provider configuration that allows developers to pass a custom nonce value. This value would override the default behavior of automatically generating and hashing a nonce with SHA256. Instead, the provided custom nonce (e.g., rawNonce) would be sent to the Apple authentication endpoint and subsequently be used in the backend for token verification.