DubboAdmin支持Dubbo3+Nacos做如下变更是否合适

### Environment

* Dubbo Admin version: 0.5.0
* Java version: 1.8
* Nacos: 2.1.0

### Steps to reproduce this issue

1. 集成Dubbo3.x应用发现问题

NacosServiceMapping的初始化逻辑注入Url为metadataUrl，参考代码ConfigServer.getServiceMapping(ServiceDiscovery serviceDiscovery, InstanceRegistryCache instanceRegistryCache)
而目前按照Dubbo的处理逻辑，metadataUrl的group默认值为dubbo、registryUrl的group默认值为DEFAULT_GROUP，即存在metadataUrl与registryUrl不一致的情况，此时NacosServiceMapping使用metadataUrl初始化则导致getAllServiceNames()结果为空，现象为管理界面服务列表为空，此处应该修改为使用registryUrl进行初始化，才能使服务列表正常获取和显示，参考如下
```
@Bean
@DependsOn("dubboRegistry")
ServiceMapping getServiceMapping(ServiceDiscovery serviceDiscovery, InstanceRegistryCache instanceRegistryCache) {
ServiceMapping serviceMapping = new NoOpServiceMapping();
if (registryUrl == null) {
return serviceMapping;
}
MappingListener mappingListener = new AdminMappingListener(serviceDiscovery, instanceRegistryCache);
serviceMapping = ExtensionLoader.getExtensionLoader(ServiceMapping.class).getExtension(registryUrl.getProtocol());
serviceMapping.addMappingListener(mappingListener);
serviceMapping.init(registryUrl);
return serviceMapping;
}
```

Start at ConfigServer.getServiceMapping(ServiceDiscovery serviceDiscovery, InstanceRegistryCache instanceRegistryCache) and inspect how NacosServiceMapping is initialized. Compare the metadataUrl and registryUrl group values, then verify that initializing with registryUrl allows the Dubbo 3 and Nacos service list to load and display correctly.

Written by the indexing model from the issue text.