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"
    }
]
```


It would be better to do this combining of endpoints somewhere before its served.
When a user wants to see the endpoints, they should be served a list of the endpoints for that service, rather than the endpoints split up by handler, because a `Service` is exposed as one service, rather than multiple, right?