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"
    }
]
```


Right, that is obviously possible. But I think the ServiceSpec should contain each individual HttpServiceSpec, since that contains details which you may want. 

Maybe better to add a new field "endpoints" with the aggregations, and let the HttpServiceSpec remain as is.