sysext/containerd: define drop-in config directory

[satwiksps]

Define containerd drop-in config directory in base configuration files.

This PR adds an imports = ["/etc/containerd/conf.d/*.toml"] directive to the default containerd configuration files shipped in the sysext overlay (config.toml and config-cgroupfs.toml).

Currently, customizing containerd settings requires users to copy /usr/share/containerd/config.toml to /etc/containerd/config.toml and override the CONTAINERD_CONFIG environment variable via a systemd drop-in. This leads to a degraded UX where users miss out on base configuration updates shipped in newer Flatcar releases.

With this change, containerd automatically loads any .toml drop-in files placed in /etc/containerd/conf.d/*.toml and merges them with Flatcar's default settings. If no drop-in files exist, containerd boots normally with default settings.

• Fixes [RFE] Define containerd drop-in config directory Flatcar#1127

Testing done

Tested on a live Flatcar Linux VM (v4593.2.4) running in VirtualBox via Vagrant:

1. Applied the updated config.toml containing imports = ["/etc/containerd/conf.d/*.toml"].

2. Created a test drop-in file at /etc/containerd/conf.d/50-custom-log.toml:

   echo 'version = 2' | sudo tee /etc/containerd/conf.d/50-custom-log.toml
   echo '[plugins."io.containerd.grpc.v1.cri"]' | sudo tee -a /etc/containerd/conf.d/50-custom-log.toml
   echo '  max_container_log_line_size = 65536' | sudo tee -a /etc/containerd/conf.d/50-custom-log.toml

3. Restarted containerd:

   sudo systemctl restart containerd

4. Output verification:

   core@localhost ~ $ sudo containerd --config /etc/containerd/config.toml config dump | grep max_container_log_line_size
       max_container_log_line_size = 65536

5. Verified backward compatibility: When no drop-in files exist in /etc/containerd/conf.d/, containerd boots cleanly without errors.

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.