Make Apple provider Secret Key optional for native-only Sign in with Apple

When using native iOS Sign in with Apple (via React Native Expo), the Secret Key is not used. The OS handles authentication and provides an id_token directly to Supabase. The Secret Key is only needed for web-based OAuth flows. However, the Apple provider settings page in the Supabase Dashboard requires a Secret Key to save any changes. This means that if you need to add a new bundle ID to the Client IDs field (e.g. when adding a new white-label app), you cannot save without also providing a Secret Key, even if you don't need one. Expected behavior: The Secret Key field should be optional, or the UI should clearly indicate it's only required for web-based OAuth. Developers using native-only Apple Sign In should be able to update Client IDs without being blocked. Context: We run multiple white-label apps (each with their own bundle ID) using native iOS Sign in with Apple. When adding a new app's bundle ID to the Client IDs list, we were blocked from saving becau…