Create examples for all kinds of plugins ORT supports

[sschuberth]

Most of ORT's tools use Java's ServiceLoader mechanism to be able to load plugins from the classpath. We should have individual code examples, hosted in a different repository, that show how to create simple analyzer (PackageManager), advisor, downloader (VersionControlSystem), scanner and reporter plugins.

[sschuberth]

Once #6602 is done, it's probably about to add a separate README.md to the plugins directory.

[sschuberth]

One way to provide such examples is via an "SDK" for a specific plugin type as done at https://github.com/oss-review-toolkit/ort-package-manager-plugin. I believe we should follow that "SDK" approach also for other plugin types.