feat(edit): configure /usr overlay

[p5]

Closes #474

Allows the operator to control the /usr overlay from bootc edit, and displays the current configuration within bootc status. Also updates the bootc usr-overlay to use the new overlay functions, but this should be non-breaking.

I've added a "best effort" functionality to umount /usr, though this seems to be an ostree thing that's not really possible to do in a smooth way - every time I tried it, I received (device busy) errors and /usr remained r/w.

---

Here's the outputs in both human and YAML output formats:

❯ sudo ./target/debug/bootc status 
● Booted image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
        Digest: sha256:b1874fbb5f442657f3cc07a547c54292ca3f24fe828ab4f3132df4c9f23f1e3e (amd64)
       Version: 250425 (2025-04-25T01:46:32Z)

  Rollback image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
          Digest: sha256:b1874fbb5f442657f3cc07a547c54292ca3f24fe828ab4f3132df4c9f23f1e3e (amd64)
         Version: 250425 (2025-04-25T01:46:32Z)

❯ sudo ./target/debug/bootc status              
[sudo] password for admin: 
  Staged image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
        Digest: sha256:b1874fbb5f442657f3cc07a547c54292ca3f24fe828ab4f3132df4c9f23f1e3e (amd64)
       Version: 250425 (2025-04-25T01:46:32Z)

● Booted image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
        Digest: sha256:b1874fbb5f442657f3cc07a547c54292ca3f24fe828ab4f3132df4c9f23f1e3e (amd64)
       Version: 250425 (2025-04-25T01:46:32Z)
  /usr overlay: read-write

  Rollback image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
          Digest: sha256:90fd7bc0675e923511a6edabb2e584f752feddbf8f90e0d610f7fc63d3e03f23 (amd64)
         Version: 250423 (2025-04-23T01:46:06Z)

❯ sudo ./target/debug/bootc status --format yaml
apiVersion: org.containers.bootc/v1
kind: BootcHost
metadata:
  name: host
spec:
  image:
    image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
    transport: registry
    signature: containerPolicy
  bootOrder: default
  usrOverlay: readWrite
status:
  staged:
    (...)
  booted:
    image:
      image:
        image: ghcr.io/rsturla/eternal-linux/lumina:42-nvidia-open
        transport: registry
        signature: containerPolicy
      version: '250425'
      timestamp: 2025-04-25T01:46:32Z
      imageDigest: sha256:b1874fbb5f442657f3cc07a547c54292ca3f24fe828ab4f3132df4c9f23f1e3e
      architecture: amd64
    cachedUpdate: null
    incompatible: false
    pinned: false
    store: ostreeContainer
    ostree:
      checksum: cac59f70910b5ef683d1e6589609789d104d206c18089efa14b51619466cfe17
      deploySerial: 0
  rollback:
    (...)
  rollbackQueued: false
  usrOverlay: readWrite
  type: bootcHost

Screencast From 2025-04-26 20-45-28.webm

As with my previous PRs, I am still extremely new to Rust, so please be harsh with the reviews. I will improve as I attempt more of these.

[p5]

This PR has been updated to also include changing the /usr overlay within bootc edit.

Overall, I'm quite pleased with how it turned out as it seems to fit the brief. Certainly learned a lot from this one, even if it is quite basic.

Please do let me know all the small (or large) things that could be done to improve this.

[p5]

Diff is quite large with this one. May pick this back up once the composefs stuff fully lands.