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"
    }
]
```


I think in this PR, it's adding a REST API for fetching the service spec. As I said before, whether we need a separate REST API for fetching a merged list of endpoints of a Service is a separate discussion (which I preferred not to have it now to keep the REST API simple).