curl --request GET \
  --url https://api-eu.flexportal.io/v1/billing-groups \
  --header 'Authorization: Bearer <token>' \
  --header 'Tenant-ID: <tenant-id>'

{
  "success": true,
  "billingGroups": [
    {
      "billingGroupId": "<string>",
      "tenantId": "<string>",
      "customerId": "<string>",
      "groupName": "<string>",
      "billingDay": 14,
      "totalMonthlyAmount": 1,
      "currency": "<string>",
      "status": "active",
      "createdBy": "<string>",
      "createdAt": "<string>",
      "updatedAt": "<string>",
      "rentalIds": [],
      "activeRentalCount": 0,
      "notes": "<string>"
    }
  ],
  "count": 123,
  "hasMore": true,
  "nextCursor": "<string>"
}