Add devices & init_format to rpi-imager.json

REF: https://downloads.raspberrypi.org/os_list_imagingutility_v3.json

As requested, since Raspberry Pi Imager (v1.8.1) new fields need to be added going forwards.

"devices" is made up about the "rpi device" and "architecture".

Supported devices can be found in "imager" -> "devices" -> "tags"

As we know the filename structure, we can extract these values.