Add example: OAuth 2.0 integration with Express

[aybanda]

Summary

I'd like to add an OAuth 2.0 integration example for Express to complement the existing FastAPI OAuth 2.0 example (oauth2-with-fastapi).

Following the pattern from oauth2-with-fastapi, I'll implement:

1. GET /login - Initiates OAuth 2.0 flow with state parameter for CSRF protection
2. GET /callback - Handles OAuth callback, exchanges code for token, retrieves user info
3. GET /api/user/{user_id} - Protected endpoint requiring OAuth authentication
4. State Management - Secure state parameter handling
5. Token Storage - Proper token handling (in-memory for demo, with notes for production)