Ensure notify when proxy connections disconnect (#65697)

TransportService doesn't respond to the pending requests of proxy
connections when the underlying connections get disconnected because
proxy connections do not override the getCacheKey method. Some CCS
requests would never be completed because of this bug.

Author: dnhatn

server/src/main/java/org/elasticsearch/transport/RemoteClusterConnection.java

@@ -371,6 +371,11 @@ public boolean isClosed() {
         public Version getVersion() {
             return proxyConnection.getVersion();
         }
+
+        @Override
+        public Object getCacheKey() {
+            return proxyConnection.getCacheKey();
+        }
     }
 
     Transport.Connection getConnection() {