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"
    }
]
```


```java
<V> Map<String, V> copy(Map<String, V> map) {
  Map<String, V> result = new HashMap<String, V>();
  result.putAll(map);
  return Collections.unmodifiableMap(result);
}
```

You might want to put this method into a `CollectionUtils` class in internal package of the cdap-api module.