SPI: message

create the message flow to payment on spi, should connect with the transition manager and send messages back to an psp

Start by locating the SPI payment flow and the transition manager, then trace how messages are currently sent to or received from a PSP. The issue provides no file or test names, so the required boundaries and message behavior need clarification before implementation. Done should mean the SPI flow connects through the transition manager and sends messages back to a PSP.

Written by the indexing model from the issue text.