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"
    }
]
```


Wouldn't that get double `//` at the beginning if the class is annotated with path prefix with "/" (e.g. "/v2")? Same apply to method annotation as well.