Changes to Listar checkouts

deleted the header request parameter X-SECRET-KEY

the endpoint scheme security secretKey was added to the API

api operation id list-checkouts removed and replaced with checkouts_listCheckouts

added the optional property items/application_fee_amount to the response with the 200 status

added the optional property items/application_fee_percent to the response with the 200 status

added the optional property items/bank_transfer_memo to the response with the 200 status