Add user.

Create a user. If you want to create an admin user, you need to send {IsAdmin: admin} in the user_permissions field e.g ```{user_permissions: {IsAdmin: admin},last_name: sasuke,email_address: [e-mail],first_name: itachi}```. You can add a user to a given user-group by sending the group_id in the request body. If you want to give a user access only to specific objects e.g policies you can send the object and the permission granted to the user (read or write permission in the user_permissions field (check the request example given for more details)).