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 get spec endpoint should be returning the spec without transformation as getting service endpoints is not the only usage of the API. Retaining the data structure of the spec is a must (treat it as a REST API for getting meta data of a service program). I expect UI will get the spec and extract necessary information from it to perform rendering that it needs (e.g. it may or may not group endpoints with handler name). However, in the cdap-client and CLI, we could have extra methods that blends all endpoints from multiple handlers into one list for convenient purpose, but it's all on client side.

I personally prefer REST API that gives unique information, rather than having multiple APIs that gives the same/similar information but in different respond data structure.