Add Response Caching Support

[jordanpartridge]

Implement basic response caching using Laravel's cache:

// In config/github-client.php
'cache' => [
    'enabled' => true,
    'ttl' => 3600, // seconds
    'store' => 'redis', // Use specific cache store
],

Implementation:

1. Add cache config
2. Create CacheMiddleware
3. Add cache key generation
4. Add cache invalidation on writes
5. Add tests

Focus on GET requests first for simplicity.