Quadlet sugar

[angelcerveraroldan]

Basic sugar for quadlets. Allows for the following syntax:

variant: fcos
version: 1.8.0-experimental
systemd:
  quadlets:
    - name: sleepy-disk.container
      rootful: true
      contents_local: containers/sleepy.container
    - name: sleepy-infinity.container
      rootful: true
      contents: |
        [Unit]
        Description=A sleepy container
        [Container]
        ContainerName=sleepy-pod-inf
        Image=quay.io/fedora/fedora
        Exec=sleep infinity
        [Install]
        WantedBy=multi-user.target

I still need to add examples for the docs, and to run more tests.

Docs about quadlets can be found here.

[gemini-code-assist]

Code Review

This pull request introduces basic support for Podman quadlets, allowing their definition within the systemd configuration. The changes include extending the Systemd struct with a Quadlets field, defining a new Quadlet struct, and implementing the translation logic to convert quadlets into Ignition files or symlinks. Comprehensive validation has been added to ensure correct quadlet naming conventions and content sources. The new functionality is well-covered by unit tests, demonstrating handling of basic quadlets, template instances, local content, and error conditions. The implementation is clean and robust for the features introduced.