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.

private final URL subscribeUrl;
        private final Invoker originInvoker;
        private List<Configurator> configurators;
        public OverrideListener(URL subscribeUrl, Invoker originalInvoker) {
            this.subscribeUrl = subscribeUrl;
            this.originInvoker = originalInvoker;
        }
        /**
         * @param urls The list of registered information, is always not empty, The meaning is the same as the
         *             return value of {@link org.apache.dubbo.registry.RegistryService#lookup(URL)}.
         */
        @Override
        public synchronized void notify(List<URL> urls) {
            logger.debug("original override urls: " + urls);
            List<URL> matchedUrls = getMatchedUrls(urls, subscribeUrl.addParameter(CATEGORY_KEY,
                    CONFIGURATORS_CATEGORY));
            logger.debug("subscribe url: " + subscribeUrl + ", override urls: " + matchedUrls);
            
            if (matchedUrls.isEmpty()) {
                return;
            }
            this.configurators = Configurator.toConfigurators(classifyUrls(matchedUrls, UrlUtils::isConfigurator))
                    .orElse(configurators);
            doOverrideIfNecessary();
        }
        public synchronized void doOverrideIfNecessary() {