[WIP] feat: add cdi-spec-dir option to top level options

[micahcc]

In response to https://github.com/containers/podman/issues/18292.

One of the primary benefits of podman is that its possible run without altering the host system. By allowing cdi directories to be passed via command line it maintains this feature.

I'm particularly interested in running podman in a nix-shell with nvidia GPU passthough. Podman and all its dependencies (including nvidia-container-toolkit) come from nix and the host system again doesn't need altering. The one problem with that right now is that I have to write /etc/cdi/nvidia.yaml to be able to use the GPU. Note that the old nvidia-runtime-hook does not work for my use case (which involves vulkan passthrough).

With this change it will be possible to generate a user nvidia.yaml and point podman at that user-specific mount instructions there.

Does this PR introduce a user-facing change?

- Add cdi-spec-dir option to insert additional CDI Spec search paths into the CDI loader.

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[micahcc]

Needs: https://github.com/containers/common/pull/1834

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