Allow overriding ServerRpcHandler creation in PushHandler (#10892)

Fixes #4537

server/src/main/java/com/vaadin/server/communication/PushHandler.java

 
     private int longPollingSuspendTimeout = -1;
 
+    private final ServerRpcHandler rpcHandler = createRpcHandler();
+
     /**
      * Callback interface used internally to process an event with the
      * corresponding UI properly locked.
         assert vaadinRequest != null;
 
         try {
-            new ServerRpcHandler().handleRpc(ui, reader, vaadinRequest);
+            rpcHandler.handleRpc(ui, reader, vaadinRequest);
             connection.push(false);
         } catch (JsonException e) {
             getLogger().log(Level.SEVERE, "Error writing JSON to response", e);
         this.service = service;
     }
 
+    /**
+     * Creates the ServerRpcHandler to use.
+     *
+     * @return the ServerRpcHandler to use
+     * @since
+     */
+    protected ServerRpcHandler createRpcHandler() {
+        return new ServerRpcHandler();
+    }
+
     /**
      * Suspends the given resource.
      *