Code duplication between composefs-rs and bootc

[Johan-Liebert1]

Currently, bootc cfs acts as a proxy to cfsctl which is basically implemented as copying the entirety of https://github.com/composefs/composefs-rs/blob/b9163a8f5a24879a1a51277c6d12f40dd074a70d/crates/cfsctl/src to some file in bootc.

One way to fix this would be to make the entire CLI api in composefs-rs public, but not sure if that's a good idea.

Another way is to always build cfsctl along with bootc and use the binary as a proxy.

[cgwalters]

We chatted about this, I think for now making cfsctl a crate with a single pub api that accepts an arg vector like is done in bootc would be simplest.

[cgwalters]

This is fixed by #2023