Add endpoint to return all the areas in the urban region of a state

**URL:** `api/v2/states/:state_id/urban/areas` or `api/v2/states/:state_name/urban/areas` or `api/v2/states/:state_code/urban/areas`
**Parameters:**  `state_id/urban/areas` or `state_name/urban/areas` or `state_code/urban/areas`.

_For more details, check https://github.com/devcenter-square/states-and-cities/wiki/List-of-Endpoints_