When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

ArgumentConfig argument = new ArgumentConfig();
        argument.setIndex(0);
        argument.setCallback(false);
        MethodConfig method = new MethodConfig();
        method.setName("echo");
        method.setArguments(Collections.singletonList(argument));
        service.setProvider(provider);
        service.setApplication(app);
        service.setRegistry(registry);
        service.setInterface(DemoService.class);
        service.setRef(new DemoServiceImpl());
        service.setMethods(Collections.singletonList(method));
        service2.setProvider(provider);
        service2.setApplication(app);
        service2.setRegistry(registry);
        service2.setInterface(DemoService.class);
        service2.setRef(new DemoServiceImpl());
        service2.setMethods(Collections.singletonList(method));
        service2.setProxy("testproxyfactory");
        delayService.setProvider(provider);
        delayService.setApplication(app);
        delayService.setRegistry(registry);
        delayService.setInterface(DemoService.class);
        delayService.setRef(new DemoServiceImpl());
        delayService.setMethods(Collections.singletonList(method));
        delayService.setDelay(100);
        ConfigManager.getInstance().clear();
    }
    @AfterEach
    public void tearDown() {