The board has a second SPI bus wired up for communication with two RFID readers
if needed.
However, the code only provided a constructor with no arguments, preventing
the use of a second instance on the different bus.
Update the constructor and the definitions to allow changing the chip select
and SPI bus used when creating an RFID reader.
Add a new class to handle the RFID module and associated data
in order to track active tags.
Update main with RFID initialization and a minimal usage to
check it is working as expected.
