chore: note central bank key integration

src/main/java/com/yau/digitalrmb/issuance/application/service/CommercialBankIssuanceApplicationService.java

@@ -34,6 +34,7 @@ import java.util.UUID;
 
 @Service
 public class CommercialBankIssuanceApplicationService {
+    // TODO: 接入“人民银行生成数字货币实验模块”后，使用该模块生成并返回的第二私钥标识，移除当前临时密钥引用。
     private static final String SIGNING_KEY_REF = "sm2-key-02";
     private static final DateTimeFormatter TIMESTAMP_FORMAT = DateTimeFormatter.ofPattern("yyyyMMddHHmmss");