Create a user

curl --request POST \
  --url https://api.proficientai.com/users \
  --header 'Content-Type: application/json' \
  --data '{
  "external_id": "<string>",
  "first_name": "<string>",
  "last_name": "<string>",
  "live_mode": true
}'

{
  "id": "<string>",
  "object": "user",
  "created_at": 123,
  "external_id": "<string>",
  "first_name": "<string>",
  "last_name": "<string>",
  "live_mode": true,
  "updated_at": 123
}