Event instance should have contextual link to edit event series

I feel this is a bit harder/weirder than it should be in Drupal but seems there is no way to alter the parameter for a contextual link (we need to get the eventseries ID from the eventinstance) and the only way to add an additional parameter is like this:

Add a src/EventInstanceViewBuilder.php with:

namespace Drupal\recurring_events;

use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
use Drupal\Core\Entity\EntityViewBuilder;

/**
 * View builder handler for event instances.
 *
 * Provide the event series ID as a parameter for a contextual link
 * to edit the series from the instance, as often event instances
 * will inherit all important information from the series.
 */
class EventInstanceViewBuilder extends EntityViewBuilder {

  /**
   * {@inheritdoc}
   */
  protected function alterBuild(array &$build, EntityInterface $entity, EntityViewDisplayInterface $display, $view_mode) {

    // Note there is no proper interface for EventInstance to reference here.
    /** @var \Drupal\recurring_events\Entity\EventInstance $entity */
    parent::alterBuild($build, $entity, $display, $view_mode);
    if ($entity->id()) {
      if ($series = $entity->getEventSeries()) {
        $build['#contextual_links']['eventinstance']['route_parameters']['eventseries' => $series->id()];
      }
    }
  }

}

}

Untested but it should be something like that.

And then all it should take is the addition of an eventseries entry with the existing eventseries route to the eventinstance group in the contextual links yaml file added in #3575563: Entities should display contextual links

Patch adds a custom EventInstanceViewBuilder that passes the event series ID to the contextual links system, a contextual link YAML definition for "Edit event series" under the eventinstance group, a configurable setting (enabled by default) at Event Instance Settings to toggle the link visibility, and template updates to support contextual links rendering. Related to #3575563.

The idea behind this issue makes sense to me.

Added a few comments regarding the implementation approach that might make the code slightly more compact.

Some tests tests to support this new feature would be highly appreciated.

Regarding the ability to enable or disable the link from the settings from , I'm not sure if this is something that the module should take care of.