Handle multiple values

Before:

 if ($value) {
            $storage = \Drupal::entityTypeManager()->getStorage($entity_type);
            $entity = $storage->load($value);
            $form['override'][$block->getDerivativeId()][$type][$id]['value']['#default_value'] = $entity;
          }

After:

 if ($value) {
            $storage = \Drupal::entityTypeManager()->getStorage($entity_type);

            $values = preg_split("/[\+,]+/", $value) ?: [];
            $default = $storage->loadMultiple($values);
            $form['override'][$block->getDerivativeId()][$type][$id]['value']['#default_value'] = $default;
          }

todo have to test mixing "+" and ",".

Right now tested "+" and "single values"

fix pushed.

Ongoing testing.

I had some issues testing your proposition.
The form configuration do not allowed me to have multiple values in the "entity_autocomplete".

So i created a patch that allow the user to add multiple entities using the "#tags" parameter.
I also add a check to only do that when the related view allow ultiple elements.

Finally i made a little change in the execute method to be able to use the multiple values in the context filter itself.

Please feel free to comment my proposition.