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

[osioke]

URL: api/v2/states/:state_id/urban/towns or api/v2/states/:state_name/urban/towns or api/v2/states/:state_code/urban/towns
Parameters: state_id/urban/towns or state_name/urban/towns or state_code/urban/towns.
Response(200)

[
  {
    id: 1, 
    state: "Ab", 
    towns: 
    [
       {id: 1, name: "Umahia"}, 
       {id: 2, name: "Aba"}
    ]
  }, 
  {...}
]

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