Add OAuth template for PHP with Google integration

[Copilot]

This PR implements a complete OAuth template for PHP developers that provides ready-to-use Google OAuth integration without requiring any configuration (except for OAuth credentials).

🔐 Features Added

OAuth Endpoints

• /oauth/login.php - Initiates OAuth flow and redirects to Google
• /oauth/callback.php - Handles OAuth callback and exchanges code for tokens
• /oauth/user.php - Returns authenticated user profile information
• /oauth/logout.php - Clears user session and logs out

Interactive Demo

• Demo page: /oauth-demo.html - Interactive interface to test OAuth flow
• API testing: Built-in buttons to test endpoints
• Setup instructions: Complete guide for Google OAuth configuration

Configuration & Dependencies

• Composer integration: Added composer.json with Google API Client library
• Configuration template: src/config/oauth.php with placeholder values
• Environment variables: .env.example for secure credential management
• Session management: Secure OAuth state handling to prevent CSRF attacks

Documentation

• Comprehensive README: Updated with OAuth setup instructions
• Security notes: Best practices for OAuth implementation
• File structure: Clear organization of OAuth components

🎯 Usage

1. Install dependencies:

composer install

2. Configure OAuth credentials in src/config/oauth.php:

'client_id' => 'your-google-client-id',
'client_secret' => 'your-google-client-secret',

3. Visit /oauth-demo.html for interactive demo

📸 Demo Screenshot

The demo page shows:

• Login with Google button
• API endpoint testing
• Complete setup instructions
• Interactive response display

🔧 Technical Implementation

• Security: CSRF protection with state parameter validation
• Error handling: Proper error responses for configuration issues
• Session management: PHP sessions for user state persistence
• Minimal changes: Added only necessary files without modifying existing functionality

The template provides a complete OAuth implementation that developers can use immediately with just credential configuration.

Fixes #1.

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