Add a reuse-lint job to CI
This CI job will check that all the files have a proper SDPX license, and that all the licenses used are included in the LICENSES folder.
You can see an example of the summary produced on this very MR: https://invent.kde.org/plasma/oxygen-sounds/-/jobs/1044660
I will abuse @ngraham patience by claiming that this small change helps the Automation goal.