rp2040: Add spi slave example

[Grazfather]

Add a SPI slave example.

This one behaves like the example linked from the spi master example, but doesn't work with the spi master example.

That example switches between 12 and 8 bit mode, which is cool, but in practice is hard to synchronize.

I'd propose dropping that part from the example so that spi master and spi slave can actually behave with each other.

Tested by removing the 12-bit part of the spi master code, as well as against the spi master pico-examples code.

TODO:

• Test spi slave on rp2350 (spi master already tested on this chip)
• Move files from rp2040-only directory