SPI / OLED Transport - Add Software CS capabilities

[stephenhensley]

This could be added directly to the SPI handle. However, with the recent change in callback structures for DMA transactions it is possible to self-manage software CS in pretty much all use cases (blocking/polling and non-blocking/DMA).

So I think this should be added to the Transport class, and it should just manage the CS pin before/after transmissions.

This would make it pretty easy to set up a few OLEDs on the same SPI bus if desired as well.