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... my bad, you might want to keep this for compatibility reason for a couple versions, but open a JIRA to have it removed in future (maybe 3.0).