Add endpoint that returns all districts in the rural region of a state

[theterminalguy]

URL: api/v2/states/:state_id/rural/districts or api/v2/states/:state_name/rural/districts or api/v2/states/:state_code/rural/districts
Parameters: state_id/rural/districts or state_name/rural/districts or state_code/rural/districts.

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