root resizefs mount point

# Bug report

When using bootc along with cloud-init, the resizefs plugin doesn't function properly due to how bootc lays out its filesystems.

Before cloud-init starts, it:
1. mounts `/`
2. bindmounts / to `/etc`,`/var`,`/sysroot` (ro)
3. mounts composefs over `/` (ro)

Then resizefs tries to resize `/` and it fails, as its composefs

For now, I've patched cloud-init to change the path from `/` to `/etc` and it works.

There needs to be a way to customize the path used for resizing the rootfs to support bootc.

Start with the cloud-init resizefs plugin and trace how it selects `/` for resizing. Reproduce the bootc mount layout described in the issue, then define and implement a configurable resize path that supports `/etc`; done means resizefs succeeds with bootc without breaking the default root path.

Written by the indexing model from the issue text.