support deploying a composefs directly

[cgwalters]

ostree has two major parts:

• OstreeRepo (object store, commits)
• OstreeSysroot (has-a OstreeRepo, extends with bootloader support, deployments, etc.)

As part of containers/bootc#20 and https://github.com/cgwalters/composefs-oci what would be quite helpful as an intermediate step is if we could try to sever some of the dependencies of OstreeSysroot on OstreeRepo.

Specifically: I'd like to be able to pass a composefs blob to ostree_sysroot_deploy_tree instead of an ostree commit. I think this wouldn't be really hard...basically early on we do:

if legacy ostree:
just call existing ostree_sysroot_get_deployment_dirpath + glnx_opendirat
else:
mount the composefs as a file descriptor (xref containers/storage#2069 (comment) )

I think there's a few corner case bits that scrape the ostree commit for metadata; maybe we provide that metadata externally for now.

There's some more work for sure here too around things like generate_deployment_refs() ...we just skip that I think in this case and require the caller to do GC?

[cgwalters]

Though this relates to containers/composefs#332 in that if we did that we'd need to teach ostree how to crack open UKIs to get access to the embedded composefs in order to do GC.