Add Mollie API integration and webhook for software upgrade payments

- Introduced the `CaBundle.php` class for managing CA certificates.
- Updated `installed.json` and `installed.php` to include the new `composer/ca-bundle` dependency.
- Added `platform_check.php` to enforce PHP version requirements.
- Created `initialize.php` for initializing the Mollie API client with the API key.
- Implemented `webhook_mollie.php` to handle webhook callbacks for software upgrade payments, including transaction status updates and invoice generation.
- Integrated DomPDF for generating invoices and sending them via email.

assets/mollie/src/Resources/IssuerCollection.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Mollie\Api\Resources;
+
+class IssuerCollection extends \Mollie\Api\Resources\BaseCollection
+{
+    /**
+     * @return string|null
+     */
+    public function getCollectionResourceName()
+    {
+        return null;
+    }
+}