- Add State enum with Closed, Open, and HalfOpen states and String method
- Add CircuitBreaker struct with mutex, state tracking, failure/success counters, and timeout
- Add Config struct with FailureThreshold, SuccessThreshold, and Timeout fields
- Add NewCircuitBreaker constructor with Config parameter
- Add DefaultConfig with 5 failure threshold, 2 success threshold, and 60 second timeout