Feature: #88818 - Introduce events to modify CKEditor configuration

Description

The following new PSR-14-based Events are introduced which allow to modify CKEditor configuration.

Example

An example implementation how you could extend the existing configuration to register a new plugin:

EXT:my_extension/Configuration/Services.yaml

services:
  Vendor\MyExtension\EventListener\RteConfigEnhancer:
    tags:
      - name: event.listener
        identifier: 'ext-myextension/rteConfigEnhancer'
        method: 'beforeGetExternalPlugins'
        event: TYPO3\CMS\RteCKEditor\Form\Element\Event\BeforeGetExternalPluginsEvent
      - name: event.listener
        identifier: 'ext-myextension/rteConfigEnhancer'
        method: 'beforePrepareConfiguration'
        event: TYPO3\CMS\RteCKEditor\Form\Element\Event\BeforePrepareConfigurationForEditorEvent

EXT:my_extension/Classes/EventListener/RteConfigEnhancer.php

namespace Vendor\MyExtension\EventListener;

use TYPO3\CMS\RteCKEditor\Form\Element\Event\BeforeGetExternalPluginsEvent;
use TYPO3\CMS\RteCKEditor\Form\Element\Event\BeforePrepareConfigurationForEditorEvent;

class RteConfigEnhancer
{
   public function beforeGetExternalPlugins(BeforeGetExternalPluginsEvent $event): void
   {
      $data = $event->getData();
      // @todo make useful decisions on fetched data
      $configuration = $event->getConfiguration();
      $configuration['example_plugin'] = [
         'resource' => 'EXT:my_extension/Resources/Public/CKEditor/Plugins/ExamplePlugin/plugin.js'
      ];
      $event->setConfiguration($configuration);
   }

   public function beforePrepareConfiguration(BeforePrepareConfigurationForEditorEvent $event): void
   {
      $data = $event->getData();
      // @todo make useful decisions on fetched data
      $configuration = $event->getConfiguration();
      $configuration['extraPlugins'][] = 'example_plugin';
      $event->setConfiguration($configuration);
   }
}