dubbo如何访问通过istio发布的grpc服务

有一些现成的grpc服务，通过istio注册暴露服务协议。

在dubbo服务中，如果访问到这些grpc服务，在官方的mesh demo中，没有找到这种场景的例子，只有dubbo-dubbo，没有dubbo-grpc。

想要达到的目标是，在dubbo客户端中，通过@DubboReference注解，就能访问到现有的grpc服务

Start by reading the official mesh demo and the existing dubbo-dubbo example, then trace how a Dubbo client uses @DubboReference. Compare that path with the existing gRPC service exposed through Istio; done means an existing gRPC service can be reached from a Dubbo client through the annotation, with a documented example or test.

Written by the indexing model from the issue text.