Composefs changes

[Johan-Liebert1]

Most of this is in prep for #1913

1. Add option to reset soft reboot state

2. Don't soft-reboot automatically
   Aligning with ostree API, now we only initiate soft-reboot if --apply
   is passed to bootc update, bootc switch, else we only prepare the
   soft reboot

3. Update image digest query format
   After bootc/commit/49d753f996747a9b1f531abf35ba4e207cf4f020,
   composefs-rs saves config in the format oci-config-sha256:.
   Update to match the same

4. composefs/update: Handle --download-only flag
   When --download-only is passed, only download the image into the
   composefs repository but don't finalize it.
   Conver the /run/composefs/staged-deployment to a JSON file and Add a
   finalization_locked field depending upon which the finalize service will
   either finalize the staged deployment or leave it as is for garbage
   collection (even though GC is not fully implemented right now).

Some commits are split from #1913

[gemini-code-assist]

Code Review

This pull request introduces several enhancements to composefs functionality. It adds an option to reset the soft reboot state and modifies the soft reboot logic to align with the ostree API, where a soft reboot is only initiated if --apply is passed. Additionally, it updates the image digest query format to match recent changes in composefs-rs. The changes are well-structured and improve the consistency and usability of the soft reboot feature. My review includes a suggestion to refactor a part of the new reset_soft_reboot function to improve code clarity and maintainability.

[gemini-code-assist]

The logic to check for the existence and mount status of nextroot can be simplified to avoid code duplication and multiple return paths. This refactoring consolidates the checks into a single conditional block, making the function's control flow more straightforward and easier to maintain.

    let nextroot = run_dir
        .open_dir_optional("nextroot")
        .context("Opening nextroot")?;

    let mounted = match nextroot {
        Some(nextroot) => {
            let is_mounted = nextroot
                .is_mountpoint(".")?
                .ok_or_else(|| anyhow::anyhow!("Failed to get mount info"))?;
            if !is_mounted {
                tracing::debug!("Nextroot is not a mountpoint");
            }
            is_mounted
        }
        None => {
            tracing::debug!("Nextroot is not a directory");
            false
        }
    };

    if mounted {
        unmount(NEXTROOT, UnmountFlags::DETACH).context("Unmounting nextroot")?;
        println!("Soft reboot state cleared successfully");
    } else {
        println!("No deployment staged for soft rebooting");
    }

    Ok(())