bootloader: add support for uki "extras"

[allisonkarlitskaya]

I skipped this when implementing the bootloader stuff, but we're gonna want it.

[travier]

Related:

• https://archive.fosdem.org/2024/events/attachments/fosdem-2024-2024-uki-addons-and-extensions-safely-extending-ukis-kernel-command-line-and-initrd/slides/22125/Uki_addons_O97iYns.pdf
• https://www.freedesktop.org/software/systemd/man/latest/ukify.html

For example for:

• console kargs: http://github.com/coreos/fedora-coreos-config/blob/testing-devel/platforms.yaml
• kernel console keyboard layout
• ignition platform & first boot kargs until we convert them to something else