feat: add Apple OAuth support with response_mode parameter

[omnimaxxing]

Add Apple OAuth Support with form_post Response Mode

This PR adds support for Apple OAuth, specifically handling the required form_post response mode when requesting name or email scopes.

Changes

• Add responseMode option to support different OAuth response modes
• Update callback endpoint to handle both GET and POST methods
• Add comprehensive Apple OAuth example
• Update documentation with Apple OAuth setup guide

Example Usage with Apple OAuth

export const appleOAuth = OAuth2Plugin({
enabled: true,
strategyName: 'apple',
useEmailAsIdentity: true,
serverURL: process.env.NEXT_PUBLIC_URL,
clientId: process.env.APPLE_CLIENT_ID,
clientSecret: process.env.APPLE_CLIENT_SECRET,
authorizePath: '/oauth/apple',
callbackPath: '/oauth/apple/callback',
authCollection: 'users',
tokenEndpoint: 'https://appleid.apple.com/auth/token',
scopes: ['name', 'email'],
providerAuthorizationUrl: 'https://appleid.apple.com/auth/authorize',
responseMode: 'form_post', // Required for Apple when requesting name/email scopes

// ... rest of configuration
})

Testing

• Tested with both Apple OAuth and existing Google OAuth
• Verified form_post response handling
• Confirmed backward compatibility with existing implementations

Documentation

• Added Apple-specific configuration examples
• Updated environment variables documentation
• Added comments explaining the form_post requirement

Breaking Changes

None. All changes are backward compatible.