Provide a bcachefs-tools library for other applications

[jelly]

For libblockdev to support bcachefs it would be great to have a library shipped with bcachefs-tools, so we don't have to rely on parsing the output of bcachefs for getting devices, filesystem usage. btrfs-progs offers libbtrfsutil which has one notable user snapper.

One example where extra work has to be done is extracting the size / usage using regex which is quite brittle. Alternatively having JSON output would also be great for scripting with jq.

For reference, this is the libbtrfsutil reference

[koverstreet]

Yep, this needs to be worked on :)