feat: Add IP-based rate limiting to charging engine with configurable RPS and burst size

- Add rate_limit module to lib.rs exports
- Add create_rate_limiter function with SmartIpKeyExtractor for IP-based limiting
- Add RATE_LIMIT_RPS environment variable with 10 requests/second default
- Add RATE_LIMIT_BURST environment variable with 20 burst size default
- Add RateLimitResponse struct with TOO_MANY_REQUESTS status code
- Add GovernorLayer with per_second quota and burst_size configuration to router

apps/charging-engine/src/lib.rs

@@ -3,6 +3,7 @@ pub mod charging;
 pub mod circuit_breaker;
 pub mod errors;
 pub mod handlers;
+pub mod rate_limit;
 pub mod routes;
 pub mod models;
 pub mod monitoring;