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/vendor/composer/platform_check.php

@@ -0,0 +1,26 @@
+<?php
+
+// platform_check.php @generated by Composer
+
+$issues = array();
+
+if (!(PHP_VERSION_ID >= 70200)) {
+    $issues[] = 'Your Composer dependencies require a PHP version ">= 7.2.0". You are running ' . PHP_VERSION . '.';
+}
+
+if ($issues) {
+    if (!headers_sent()) {
+        header('HTTP/1.1 500 Internal Server Error');
+    }
+    if (!ini_get('display_errors')) {
+        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
+            fwrite(STDERR, 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . implode(PHP_EOL, $issues) . PHP_EOL.PHP_EOL);
+        } elseif (!headers_sent()) {
+            echo 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . str_replace('You are running '.PHP_VERSION.'.', '', implode(PHP_EOL, $issues)) . PHP_EOL.PHP_EOL;
+        }
+    }
+    trigger_error(
+        'Composer detected issues in your platform: ' . implode(' ', $issues),
+        E_USER_ERROR
+    );
+}