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"
    }
]
```


I see your point. 

Having said that, this endpoint should at least return the spec as it is in the code (since the HttpServiceSpec contains other useful information) - which would mean individual specs for individual handlers. 

If you want to have an endpoint for which lists all Service endpoints in a single list, that would have to be a separate endpoint (like it was originally). Or.. when we serialize/deserialize a ``ServiceSpecification`` in ``ServiceSpecificationCodec``, we artificially create a new field "endpoints" where we add the combination. I don't necessarily like this since we duplicate information sent back in the JSON.

@chtyim Thoughts? What would be better?