composefs: expose CLI for installing boot binaries and write composefs state

This is a feature request for exposing a CLI for installing boot binaries and write composefs state in bootc composefs backend.

bootc does not support installation of images across architectures. The main challenge of cross-installation is that bootc does not have a well-defined way of installing bootloaders across archs.

However I feel that as long as bootloader installation is handled externally (via tools or even manually by user), and the bootloader supports UAPI.1 BLS interface, there is nothing remaining in the logic of composefs backend that forbids it from being cross-arch. Therefore, exposing bootc composefs backend's boot binary installation and composefs state writing procedure can enable external integrations that make cross-arch installation possible.

Operations on composefs repo itself is already exposed via `cfsctl` (or `bootc internals cfs` that points to the same interface), so the functions that would be needed externally are `setup_composefs_bls_boot`, `setup_composefs_uki_boot` and `write_composefs_state`