Add endpoint that returns all lgas in the rural region

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