SPI: message builder

rebuild message header from spi to psp, to prevent leak of informations of other psp

Locate the SPI-to-PSP message builder and the code that constructs the message header. First trace which information crosses that boundary, then check for existing tests around message construction. Done means the PSP header no longer exposes information belonging to other PSPs and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.