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`).