install: Support /usr/lib/bootc/install-to-disk executable

One thing to consider: Support the container image to shipping its own default `to-disk` installer that sets up partitioning etc. that is an executable in `/usr/lib/bootc/install-to-disk` for example.

This would be clearer actually if we more cleanly severed the existing "to-disk" and "to-filesystem" code here, and we ship the `to-disk` setup as `/usr/lib/bootc/install-to-disk.default` or something?

Also xref https://github.com/containers/bootc/issues/437 where I think we need configurability for what the base image supports in general.

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Alternatively to this, what I think could work well is just recommending that folks add their own `bootc install` wrapper in the container image e.g. `/usr/bin/exampleos-installer` that sets up partitioning using whatever CLI and configfiles it wants, then just invokes `bootc install to-filesystem`. Then they can also drop in a config file that disables `bootc install to-disk` entirely.