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.

if (INSTANCE == null) {
            ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(REGISTRY_PROTOCOL); 
        }
        return INSTANCE;
    }
    
    private static String[] getFilteredKeys(URL url) {
        Map<String, String> params = url.getParameters();
        if (CollectionUtils.isNotEmptyMap(params)) {
            return params.keySet().stream()
                    .filter(k -> k.startsWith(HIDE_KEY_PREFIX))
                    .toArray(String[]::new);
        } else {
            return new String[0];
        }
    }
    public void setCluster(Cluster cluster) {
        this.cluster = cluster;
    }
    public void setProtocol(Protocol protocol) {
        this.protocol = protocol;
    }
    public void setRegistryFactory(RegistryFactory registryFactory) {
        this.registryFactory = registryFactory;
    }
    public void setProxyFactory(ProxyFactory proxyFactory) {
        this.proxyFactory = proxyFactory;
    }
    @Override
    public int getDefaultPort() {