- Add CircuitState enum with Closed, Open, and HalfOpen states
- Add CircuitBreakerError enum wrapping Open state and inner errors
- Add CircuitBreaker struct with failure_count, failure_threshold, timeout, and last_failure_time fields
- Add execute method with async operation wrapper and state-based error handling
- Add check_state method with automatic Open to HalfOpen transition after timeout
- Add on_success method
