Add endpoint that returns one state

[osioke]

URL: api/v2/states/:state_id or api/v2/states/:state_name or api/v2/states/:state_code
Parameters: state_id or state_name or state_code
reponse(200):
E.g: curl -X GET /api/v2/states/ab

{
 id: 1, 
 code: "AB", 
 name: "Abia", 
 capital: "Umuahia",
 lat: 5.4308921,
 lng: 7.5247243,
 zone_id: 4
}

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