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"
    }
]
```


You can remove all the codec (Service spec, HttpService spec, ServiceWorker spec) as Gson knows how to serde all POJO types.