Investigate alignment/intersection with systemd-repart

Let's look at [systemd repart](https://www.freedesktop.org/software/systemd/man/latest/systemd-repart.service.html) and our intersections with it.

A big picture overall thing here is that `bootc install` defaults to having one rootfs and inherently can work on top of any filesystem or block storage. A key thing is that the operating system size is dynamic, not a fixed partition A/B setup.

systemd-repart is definitely oriented towards DDIs and the discoverable partition spec and less so to this path, but it is obviously very general.

I think a baseline pattern that would be neat to support is injecting repart configs into the base container image to define a distinct `/var` partition.

A great first target would be e.g. initializing EBS instance store volumes in AWS, or the NTFS partition in Azure.

(Note, it looks like the service is designed to run from the initrd, so we'll want to document either regenerating or more ideally *appending* to the initrd)

Start with the linked systemd-repart documentation and the bootc install and initrd paths. Compare how repart could define a distinct /var partition and initialize AWS EBS instance-store or Azure NTFS volumes. Done should be a decided integration approach, including whether initrd configuration is regenerated or appended, with the resulting behavior documented.

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