feat: Add API key authentication support to charging engine client

- 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

apps/api-server/internal/config/config.go

@@ -19,6 +19,7 @@ type Config struct {
 type ChargingEngineConfig struct {
 	BaseURL string        `json:"base_url"`
 	Timeout time.Duration `json:"timeout"`
+	APIKey  string        `json:"api_key"`
 }
 
 type PaymentConfig struct {