Add getter setter for settings in upload service

[einpraegsam]

This feature allows to change settings when using event UploadServicePreflightEvent.

In my case I want to change allowed file types for upload per Plugin.

Usage would be somehting like:

<?php

namespace In2code\In2template\Event\Listener;

use In2code\Powermail\Events\UploadServicePreflightEvent;

class AllowImageUploadEventListener
{
    private array $imageExtensions = [
        'gif',
        'jpeg',
        'jpg',
        'png',
        'svg',
        'webp',
    ];

    public function __invoke(UploadServicePreflightEvent $event): void
    {
        $settings = $event->getSettings();
        if ($this->isFormForStoringNews($settings)) {
            $settings['misc']['file']['extension'] = implode(',', $this->imageExtensions);
            $event->setSettings($settings);
        }
    }

    public function isFormForStoringNews(array $settings): bool
    {
        return ($settings['main']['saveasnews'] ?? '') === '1';
    }
}