Support for filesystem types for non-root partitions

Related to https://github.com/osbuild/bootc-image-builder/issues/407:
I was curious if there are plans to support defining the filesystem for partitions other than root (specifically `/boot`). Currently, the way BIB works, we have a situation where users can control the filesystem type of their root partition, but have no control over `/boot` and it creates a mismatch which, while not invalid, can be a bit unusual.

If not, if we don't want to overcomplicate the bootc install configuration, I think it might be more appropriate to have pre-defined partitioning schemes that control multiple partitions (similar to Anaconda kickstart's `autopart`).

Beginne damit, das verwandte Issue #407 zu lesen, und kläre, ob das Ziel die Auswahl des Dateisystems pro Partition oder vordefinierte Partitionierungsschemata sind. Als abgeschlossen gilt die Arbeit, wenn ein abgestimmtes Konfigurationsdesign zur Steuerung von Nicht-Root-Partitionen wie /boot vorliegt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.