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"
    }
]
```


The fact that different handlers are different classes, hence they need to have their own spec so that datasets/properties can be restored individually at runtime. Since the Service spec is the root, it needs to carries individual handler specs.