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"
    }
]
```


A ServiceSpecification doesn't need to contain HttpServiceSpecifications, right?

When constructing the ServiceSpecification, transform the HttpServiceSpecification, and extract what you need: (drop the handler names which are not otherwise exposed from the Service, and aggregate the endpoints).