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"
    }
]
```


in fact, no need to be a method, just do it in one line:

```java
this.handlers = Collections.unmodifiableMap(new HashMap<String, HttpServiceSpecification>(handlers));
```