Implement Scheduler plugin for event series
This will allow users to schedule future publication of event series. These are custom entities and thus not supported by the Scheduler module out of the box.
Version 2.x of the Scheduler module supports scheduling of other entities through plugins.
This adds:
- A plugin for integrating event series from the Recurring Events module with the Scheduler module
- A class with events which are emitted when Scheduler updates Recurring Events event series
Intentionally the plugin does not deal with event instances. The Recurring Events module already ensures that the publication status of event instances follows the publication status of the corresponding series.
The actual implementation is inspired by the corresponding code for Paragraphs. https://git.drupalcode.org/project/scheduler/-/blob/6e39fbfdc18bc2a4d3b6a05b268886eafa373ffa/src/Plugin/Scheduler/ParagraphScheduler.php
Closes #3178696