WIP: Add support for configmaps

[cgwalters]

This is part of #22

[openshift-ci]

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[cgwalters]

I was typing this up, but thinking about it...perhaps it should land in ostree-ext so it can be shared with rpm-ostree?

Or...maybe we publish a crate for bootc and pull that into rpm-ostree too...

[cgwalters]

OK what I'm getting hung up on a bit here is exactly what the API is to upload objects outside of kubernetes. It looks like podman play kube basically punts on this and you point the command at an arbitrary directory...which is rather un-opinionated - and also has some annoying ergonomics because then you typically want to delete the files after they've been created.

The typical systemd-style pattern of supporting e.g. /run/bootc/api and /etc/bootc/api (but probably not /usr/lib/bootc/api since that gets weird around mutability) is perhaps a bit better than an arbitrary directory.

We'd want to have bootc api get configmap to dump all the configmap objects that bootc knows about. (Or maybe just bootc get?)