Customization (ssh keys, wifi...) not working with Raspberry Pi Imager 2.0

Looks like they just released a new version of rpi imager: https://www.raspberrypi.com/news/a-new-raspberry-pi-imager/

There is a "feature" in the new imager where it doesn't make assumptions about the OS on custom .img files like those used by raspOVOS.

Full details are at https://github.com/raspberrypi/rpi-imager/issues/1302 but I think we probably want to add a .json file describing how to customize a raspOVOS image to this repository, and then update the instructions to have users edit their "content repository" to point to the JSON file.

Seems like the file to update would be at https://github.com/raspberrypi/rpi-imager/blob/main/doc/schema-notes.md

For now, downgrading to the previous version of Raspberry Pi imager works fine (via https://github.com/raspberrypi/rpi-imager/releases/tag/v1.9.6). But presumably at some point this will need a fix.

Start by reading Raspberry Pi Imager's doc/schema-notes.md and the linked rpi-imager issue 1302 to understand the JSON needed for custom raspOVOS images. Add the JSON file and update the repository's Imager instructions to use its content repository; verify that customization works with Imager 2.0, while the v1.9.6 downgrade remains the fallback.