[lib][spi][srv] Split SPI batch unpacking into a separate library

Expose lib/spi/srv function to unpack SPI requests from a memory buffer.
This routine is used by SPI host device to parse incoming SPI requests.

We also need to separate this functionality into a separate file for
host compilation.

Bug: 118762930
Change-Id: Iaee687a1c8f1b124f9ee99cb4d1518f44cc927e6
6 files changed