Generate json file for circuitpython.org that rpi-imager can use #5632
Description
With a .json on circuitpython.org we could have CircuitPython in the rpi-imager menu for easy discovery. This json has unzipped file size and sha256s that we'd need to generate at build time.
Example file: http://unofficialpi.org/rpi-imager/rpi-imager-octopi.json