refactor(proxy): 优化证书变更源的引用

- 将 FileCertChangeSource 的具体实现细节封装起来
- 使用 CertChangeSource接口替代具体的 FileCertChangeSource 类
- 提高了代码的可维护性和可扩展性

Signed-off-by: Async <raisinata@foxmail.com>

Author: EnableAsync

proxy/src/main/java/org/apache/rocketmq/proxy/ProxyStartup.java

@@ -44,6 +44,7 @@
 import org.apache.rocketmq.proxy.processor.DefaultMessagingProcessor;
 import org.apache.rocketmq.proxy.processor.MessagingProcessor;
 import org.apache.rocketmq.proxy.remoting.RemotingProtocolServer;
+import org.apache.rocketmq.proxy.service.cert.CertChangeSource;
 import org.apache.rocketmq.proxy.service.cert.FileCertChangeSource;
 import org.apache.rocketmq.proxy.service.cert.TlsCertificateManager;
 import org.apache.rocketmq.remoting.protocol.RemotingCommand;
@@ -78,7 +79,7 @@ public static void main(String[] args) {
 
             MessagingProcessor messagingProcessor = createMessagingProcessor();
 
-            FileCertChangeSource fileCertChangeSource = new FileCertChangeSource(
+            CertChangeSource fileCertChangeSource = new FileCertChangeSource(
                 ConfigurationManager.getProxyConfig().getTlsCertPath(),
                 ConfigurationManager.getProxyConfig().getTlsKeyPath());
             TlsCertificateManager tlsCertificateManager = new TlsCertificateManager(fileCertChangeSource);