Support booting from rootfs acquired via HTTP (#36314)

This extends systemd-import-generator to not only download a disk image
at boot, but also attach it to a loopback device, so that we can boot
from it.

We have most of the pieces already in place, this just polishes some
things, to make this round.

The topmost commit contains example command lines that just work to make
`systemd-vmspawn` boot from a `mkosi serve` call.

Note that this does not address how to get the UKI running on the target
system, this only deals with the later boot phase once the UKI is
already running.

This is WIP, because it lacks docs, and I want to do some more
polishing. But it works great.

Ultimate goal, provide a complete solution so that we also can do uefi
http boot for ukis