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"
    }
]
```


Ok, I did not know that there was an internal requirement for the spec to retain the individual handler specs. That's fine then.
But shouldn't the endpoints that a service exposes be an aggregated list, rather than a mapping from handler name to endpoint?