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/MethodPrice.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Mollie\Api\Resources;
+
+class MethodPrice extends \Mollie\Api\Resources\BaseResource
+{
+    /**
+     * The area or product-type where the pricing is applied for, translated in the optional locale passed.
+     *
+     * @example "The Netherlands"
+     * @var string
+     */
+    public $description;
+    /**
+     * The fixed price per transaction. This excludes the variable amount.
+     *
+     * @var \stdClass An amount object consisting of `value` and `currency`
+     */
+    public $fixed;
+    /**
+     * A string containing the percentage being charged over the payment amount besides the fixed price.
+     *
+     * @var string An string representing the percentage as a float (for example: "0.1" for 10%)
+     */
+    public $variable;
+}