Add a customization for `sysconfig/desktop` and `sysconfig/livesys`.

[supakeen]

All desktop spins change the sysconfig/desktop and sysconfig/livesys stage. It'd be much nicer if this could be done through a customization directly.

• [x] Add osbuild stage: https://github.com/osbuild/osbuild/pull/1345
• [x] Add images support for stage: https://github.com/osbuild/images/pull/25
• [ ] Add customization to images: https://github.com/osbuild/images/pull/30
• [ ] Add customization to osbuild-composer: https://github.com/osbuild/osbuild-composer/pull/3558
• [ ] Document customization in guides: https://github.com/osbuild/guides/pull/136

[supakeen]

Note that there is discussion on the previous issue in osbuild: https://github.com/osbuild/osbuild/issues/1339#issuecomment-1634207763.

[supakeen]

During todays discussion at the team meeting it was decided that the custom files customization will be used instead of adding customizations for each configuration file(group).