This pull request replaces the jvm-example plugin (from the jvm/site plugins era) by two new plugins: a custom-settings that shows how to register and use custom settings (including secured settings) in a plugin, and rest-handler plugin that shows how to register a rest handler.
The two plugins now reside in the plugins/examples project. They can serve as sample plugins for users, a special attention has been put on documentation. The packaging tests have been adapted to use the custom-settings plugin.
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