feat: add argocd-image-updater plugin

[thatmlopsguy]

Summary

Description:

• Tool repo URL: https://argocd-image-updater.readthedocs.io/en/stable/
• Plugin repo URL: https://github.com/thatmlopsguy/asdf-argocd-image-updater

Checklist

• [x] Format with scripts/format.bash
• [x] Test locally with scripts/test_plugin.bash --file plugins/argocd-image-updater
• [x] Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI