Create a customer or upsert by external_id

{  "object": "customer",  "id": "string",  "external_id": "string",  "name": "string",  "country_key": "string",  "is_person": true,  "send_method": "email",  "street": "string",  "zip_code": "string",  "city": "string",  "phone": "string",  "email": "string",  "webpage": "string",  "att_person": "string",  "vat_number": "string",  "se_number": "string",  "p_number": "string",  "ean_number": "string",  "use_cvr": true,  "company_type_key": "string",  "preferred_invoice_language_key": "string",  "preferred_invoice_currency_key": "string",  "external_reference": "string",  "payment_condition_type": "string",  "payment_condition_number_of_days": -9007199254740991,  "invoice_mail_out_option_key": "string",  "payment_mean": "string"}