Add support for io.mkosi.uki artifact

[Cornelicorn]

This artifact would allow building the UKI inside the image (pretty much like it was done "in the old days" / pre v15) using something like kernel-install or whatever the user pleases, but especially also with mkosi-initrd.

When the images being generated are still meant to be used by the host later on, this is useful to keep variation between the UKI produced in the mkosi build and the UKIs produced on the host after kernel upgrades to a minimum by using the same tools not only for the initrd creation (as is possible using io.mkosi.initrd) but also bundling the initrds.

If io.mkosi.uki/*.efi is found, these files should be picked up as UKIs and copied into the ESP.

As far as I can see a different approach to handle this would be reworking UnifiedKernelImages= so you can disable generation of UKIs and Type 1 entries alltogether.