Add update hooks

[cgwalters]

One thing that rpm-ostree has is that by default it displays the rpm database diff between two OS images.

I think we could add something like:

/usr/lib/bootc/diff-hooks.d

That we'd run each time a deployment changes. Then software like dnf could drop in a /usr/lib/bootc/hooks.d/dnf that would basically do what rpm-ostree db diff does; it would get passed two filesystem roots (as file descriptors or maybe easier by running as a container with /from and /to mount points for the two roots).

This would get us into a space where typing bootc upgrade would at least have that CLI sugar for admins.

But I'd probably say that we shouldn't try to scope creep this package/container integration too much beyond that; if one wants to do things with packages one would invoke apt/dnf directly, which should learn how to interact with bootc.

See e.g. coreos/rpm-ostree#4924 which is about codifying an API in rpm-ostree for these diffs. I don't think we should do anything like that here; the API for such a thing should live in a tool like apt/dnf which are about packages.

IOW there'd be e.g. dnf bootc diff perhaps. (But also, dnf podman diff <imageid> <imageid>!).

Basically a core tradeoff in rpm-ostree is that we didn't invest enough in the package-independent ostree tool, and I want bootc to feel first class for non-packaged content too, and deferring package things to package tools.