- Add validateURL method to ChargingEngineClient with private IP blocking and domain whitelisting
- Add SSRF validation to CheckCredit, DeductCredit, AddCredit, GetBalance, and HealthCheck endpoints
- Add errors, net, url, and slices imports to charging_client.go
- Update HealthCheck endpoint from /health to /v1/health for consistency
- Add allowed domains list including localhost, 127.0.0.1, charging
- Add APIKey field to ChargingEngineConfig struct
- Add apiKey field to ChargingEngineClient struct
- Add setAPIKeyHeader method to set X-API-Key header when API key is configured
- Call setAPIKeyHeader in CheckCredit, DeductCredit, AddCredit, GetBalance, and HealthCheck methods
- Add ChargingEngineClient with base URL and HTTP client with configurable timeout
- Add CheckCredit to verify subscriber credit availability by IP with bytes requested
- Add DeductCredit to deduct bytes from subscriber credit balance
- Add AddCredit to add bytes to subscriber credit balance
- Add GetBalance to retrieve current credit balance for subscriber by IP
- Add HealthCheck to verify charging engine availability