Register or get customer

curl --request POST \
  --url https://api.auctionnow.io/api/alpha/merchant/customers \
  --header 'Authorization: <authorization>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "data": {
    "email": "jsmith@example.com",
    "name": "<string>"
  }
}
'

{
  "data": {
    "_id": "<string>",
    "_creationTime": 123,
    "email": "<string>",
    "name": "<string>",
    "userId": "<string>",
    "stripeCustomerId": "<string>"
  },
  "timestamp": 123,
  "message": "<string>"
}