feat(registry): 添加 Registrar 接口

- 新增 Registrar 接口，继承了 Registrar 和 Discovery接口

registry/registry.go

@@ -0,0 +1,8 @@
+package registry
+
+import "github.com/go-kratos/kratos/v2/registry"
+
+type Registrar interface {
+	registry.Registrar
+	registry.Discovery
+}