This PR includes an end-to-end example project which demonstrates the
use of the FFI. This complements [the FFI
tutorial](https://jax.readthedocs.io/en/latest/ffi.html) by putting all
of the code in one place, as well as demonstrating how FFI extensions
can be packaged. Alongside the example project, I have also added a new
GitHub Actions workflow to test the example as part of CI.
For now, the tests only run on CPU, but once we have GPU runners for
GitHub Actions (soon!), I plan on migrating the custom call examples
from `docs/gpu_ops` and `docs/cuda_custom_call` into this test case.
Similarly, I wanted to start small and this example project only
includes exactly the same functions as the tutorial for now, but I think
this could be a good place to showcase more advanced examples (including
custom calls with state).
