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"
    }
]
```


No need to say "Exposed" since it is stored in the spec and spec is always exposed. Should be more like `HttpEndpoint` as it is http specific. In future, if we have exposed (public) and non-exposed (private) endpoints, they should both be represented with this class, just that they'll belongs to different field (e.g. public in Service spec, private ones in corresponding worker spec).