Support optional update payloads, notably for ARM/AArch64 firmwares

For Raspberry Pi and other AArch64 systems, we usually need some firmware files to be installed either in the ESP, or at fixed offsets on the disk. Some firmwares are usually specific to each board and can not all be installed at the same time on an image. The installation step for those devices is also often performed from another system and not done on the system itself so we can not guess at runtime which firmwares we should install depending on the installation environment.

Thus we need a mechanism to select which one bootupd would install when setting things up when called by bootc install (directly or via bootc-image-builder with a new option) or by Anaconda.

We would then allow users to extend this list of firmwares via a container build in the case of Bootable Containers using a mechanism similar to https://github.com/coreos/bootupd/issues/766 and https://github.com/coreos/bootupd/issues/926.

This would let users build their own images with custom firmware files, build their image with bootc-image-builder, select their custom board variant, and then install it on a device for direct use.

This would provide an alternative user experience to the existing flows from:

• https://docs.fedoraproject.org/en-US/fedora-coreos/provisioning-raspberry-pi4/
• https://github.com/fedora-arm/arm-image-installer/

See previous discussions:

• https://github.com/coreos/bootupd/pull/935#issuecomment-2991545407

Begin by reviewing the linked discussions, especially bootupd#766, bootupd#926, and pull request #935, alongside the existing Fedora CoreOS and arm-image-installer flows. Done means defining and implementing a mechanism for selecting optional, board-specific firmware payloads during bootc, bootc-image-builder, or Anaconda installation, including a way to extend payloads in container builds.