Start hosted checkout for Pro upgrade

curl --request POST \
  --url https://api.wazapin.com/v1/settings/billing/upgrade \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "checkout_url": "<string>",
    "payment_session_id": "<string>",
    "invoice_id": "<string>",
    "invoice_number": "<string>",
    "reference_id": "<string>"
  }
}