Add endpoint that returns all the streets in the urban region of a state

[theterminalguy]

URL: api/v2/states/:state_id/urban/streets or api/v2/states/:state_name/urban/streets or api/v2/states/:state_code/urban/streets
Parameters: state_id/urban/streets or state_name/urban/streets or state_code/urban/streets.

For more details, check https://github.com/devcenter-square/states-and-cities/wiki/List-of-Endpoints