support ostree-ext tar mode

[cgwalters]

Today https://github.com/ostreedev/ostree-rs-ext/ implements effectively a new repository mode. The main thing is that we split off xattrs into a separate content-addressed file; partly because I was uncertain about xattrs making it through tar transformations.

We should support all ostree operations here, both reads and commits. This will involve some code duplication with ostree-rs-ext unfortunately due to the Rust/C problem. Or...in theory we could try to expose a hidden but stable C API from ostree-rs-ext as a C shared library...but, eek.

[lucab]

Perhaps without going into the uncharted territory of linking the C and Rust codebases together, for the moment it could be enough to investigate a "metadata read-only" mode purely in C here. As an intermediate step, that would allow basic handling/introspection through libostree and most of the metadata command to work on such repo (ls, log, show, fsck, etc). Writing and checkout would still live on the Rust extension only, at least for the moment.

[cgwalters]

Having it be read-only would definitely be a useful start.