Added new endpoint to create a user using REST API

Added REST API endpoint to create a user.

POST /users
Content-Type: application/json

Example request body:

{
   "username": "xh2bdi65tqbgykclu3glt7g9j",
   "email": "mikunakano@gmail.com",
   "country": "JP",
   "gender": "FEMALE",
   "birthdate": "2002-05-14",
   "sonata_id": "xh2bdi65tqbgykclu3glt7g9j"
}

Implementation of https://github.com/Project-Sonata/Profiles/issues/13

Project-Sonata / Profiles

Added new endpoint to create a user using REST API #18