Migrate bootc dependency from containers image proxy to podman

Currently, bootc relies on [containers image proxy crate](https://github.com/containers/containers-image-proxy-rs) to pull images. However, [looking at the shortcomings](https://github.com/containers/bootc/issues/147#issuecomment-1828751377) of the container image proxy, we have decided to replace it with podman.

This issue is to track the required effort.

/cc @vrothberg @cgwalters

Start by locating the bootc dependency and integration points for the containers image proxy crate; the issue names no files or tests. Review the linked shortcomings and determine the affected image-pull paths, then verify that the proxy dependency is replaced by podman and image pulls still work.

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