Demonstrate Rust code from sysroot

[cgwalters]

This is a demo/prototype for starting to add new Rust code only for OstreeSysroot.

#2427 (comment)

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[cgwalters]

I started thinking about this and couldn't help trying it out; it was easier than I thought.

However, a few obvious things:

• We definitely want to at least use https://github.com/mozilla/cbindgen; and even with that, error handling still sucks. Maybe we can teach cbindgen about GError...
• The dynamic linking thing will suck for ergonomics

[cgwalters]

Other binding tool options:

• https://rust-diplomat.github.io/book/intro.html (needs investigation, but claims some C support)
• https://cxx.rs/ - used in rpm-ostree, it's good but the C++ requirement is a hump, but the main annoyance is that it really wants to use exceptions for errors

[cgwalters]

[2023-09-20T19:41:39.924Z] ostree: failed to initialize libostreesysrs.so: /usr/libexec/libostree/libostreesysrs.so: cannot open shared object file: No such file or directory

This is because the supermin tooling only knows how to find things shipped as an RPM.

(Hmmm....maybe we could figure out some way to actually ship this thing as part of libostree-1.so...)

Or I guess, the more I think about it, pull off the band aid and just hard require Rust.