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.

ProviderConfig provider = new ProviderConfig();
        ServiceBuilder builder = new ServiceBuilder();
        builder.provider(provider);
        Assertions.assertSame(provider, builder.build().getProvider());
    }
    @Test
    void providerIds() {
        ServiceBuilder builder = new ServiceBuilder();
        builder.providerIds("providerIds");
        Assertions.assertEquals("providerIds", builder.build().getProviderIds());
    }
    @Test
    public void generic() throws Exception {
        ServiceBuilder builder = new ServiceBuilder();
        builder.generic(GENERIC_SERIALIZATION_DEFAULT);
        assertThat(builder.build().getGeneric(), equalTo(GENERIC_SERIALIZATION_DEFAULT));
        builder.generic(GENERIC_SERIALIZATION_NATIVE_JAVA);
        assertThat(builder.build().getGeneric(), equalTo(GENERIC_SERIALIZATION_NATIVE_JAVA));
        builder.generic(GENERIC_SERIALIZATION_BEAN);
        assertThat(builder.build().getGeneric(), equalTo(GENERIC_SERIALIZATION_BEAN));
    }
    @Test
    public void generic1() throws Exception {
        Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceBuilder builder = new ServiceBuilder();
            builder.generic("illegal").build();
        });
    }
    @Test
    public void Mock() throws Exception {