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"
    }
]
```


Why is it necessary that this be map -- even though there should only be one HttpService per `Service`?

Or is it the case that these correspond to the potential multiple handlers, before it is combined into one HttpService?