add plugin loader support

src/ZM/Annotation/AnnotationHandler.php

@@ -127,14 +127,18 @@ class AnnotationHandler
         // 由于3.0有额外的插件模式支持，所以注解就不再提供独立的闭包函数调用支持了
         // 提取要调用的目标类和方法名称
         $class = $v->class;
-        $target_class = new $class();
         $target_method = $v->method;
+        if ($class !== '') {
+            $target_class = new $class();
+            $callback = [$target_class, $target_method];
+        } else {
+            $callback = $target_method;
+        }
         // 先执行规则，失败就返回false
         if ($rule_callback !== null && !$rule_callback($v)) {
             $this->status = self::STATUS_RULE_FAILED;
             return false;
         }
-        $callback = [$target_class, $target_method];
         try {
             $this->return_val = middleware()->process($callback, ...$args);
         } catch (InterruptException $e) {