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"
    }
]
```


Hum.. since data is already in the Spec and I believe we do have an endpoint for getting a Service spec, then why we need a separate endpoint for just list all service endpoints?