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"
    }
]
```


To be more correct, you should use the `@HttpMethod` annotation on the method annotation to extract the actual name of the HTTP method. If you look at the `@GET` class, it looks like this:

```java
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@HttpMethod(HttpMethod.GET)
@Documented
public @interface GET {
}
```

Which the `@HttpMethod.value()` gives the string representing the actual HTTP method.