CDAP-412 REST endpoint to expose Service methods

Endpoint: ``/apps/{app-id}/services/{service-id}/endpoints-exposed``

Example output: 
```
[
    {
        "method": "GET", 
        "path": "/v1/product/{id}/catalog"
    }
]
```


hum.. this bring an interesting case. I can actually write a handler that extends from another handler that has `@Path` annotation in the parent class. E.g.

```java
@Path("/base")
class BaseClass {
  ...
}

class SubClass extends BaseClass {
  @GET
  public void get(...) {
  }
}
```

Which I believe it is supported currently and can route correct when a request is made to `GET /base`