core/dracut/ignition-ostree: add ignition-ostree-sysusers.service

[HuijingHei]

Based on #774.

Copy text here:
This introduces a new ignition-ostree-sysusers.service, which takes
care of poulating users and groups on the target sysroot before the
Ignition files stage.

[jlebon]

LGTM overall, though could we add an external test that tries to create a file owned by a user in a sysusers.d dropin? (E.g. zincati)

[HuijingHei]

could we add an external test that tries to create a file owned by a user in a sysusers.d dropin? (E.g. zincati)

Thanks @jlebon for the review!

Do you mean like following (use dnsmasq instead, as rhcos also includes this user)?
The interesting thing is the config works well without new ignition-ostree-sysusers.service, see #2670

$ cat config.bu
variant: fcos
version: 1.4.0
storage:
  files:
    - path: /etc/dnsmasq/config.d/00-dummy-placeholder.toml
      mode: 0644
      user:
        name: "dnsmasq"
      contents:
        inline: |
          # Dummy placeholder

$ cosa run --qemu-image fedora-coreos-38.20231009.20.0-qemu.x86_64.qcow2 -m 4096 -B config.bu

[core@cosa-devsh ~]$ ls -al /etc/dnsmasq/config.d/00-dummy-placeholder.toml
-rw-r--r--. 1 dnsmasq root 20 Oct 22 13:19 /etc/dnsmasq/config.d/00-dummy-placeholder.toml

[HuijingHei]

@jlebon I am not sure how I can add special test for this patch, as all systemd sysusers are generated during build process, any pointers? Thanks!