List customer's payment methods

curl --request GET \
  --url https://api.notchpay.co/customers/{id}/payment_methods \
  --header 'Authorization: <api-key>'

{
  "code": 200,
  "status": "OK",
  "message": "Payment methods retrieved",
  "items": [
    {
      "id": "pm_123456789",
      "type": "Mobile Money",
      "channel": "cm.mtn",
      "phone": 237680000000,
      "created_at": "2023-01-01T12:00:00.000Z"
    }
  ]
}