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.

initialize.php

@@ -0,0 +1,18 @@
+<?php
+/*
+ * Make sure to disable the display of errors in production code!
+ */
+ini_set('display_errors', '1');
+ini_set('display_startup_errors', '1');
+error_reporting(E_ALL);
+
+// Include Mollie functions
+require_once __DIR__ . "/assets/mollie/vendor/autoload.php";
+
+/*
+ * Initialize the Mollie API library with your API key.
+ *
+ * See: https://www.mollie.com/dashboard/developers/api-keys
+ */
+$mollie = new \Mollie\Api\MollieApiClient();
+$mollie->setApiKey(mollie_api_key);