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/ClientLink.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Mollie\Api\Resources;
+
+use Mollie\Api\Types\ApprovalPrompt;
+class ClientLink extends \Mollie\Api\Resources\BaseResource
+{
+    /**
+     * @var string
+     */
+    public $resource;
+    /**
+     * Id of the client link.
+     *
+     * @example csr_vZCnNQsV2UtfXxYifWKWH
+     * @var string
+     */
+    public $id;
+    /**
+     * An object with several URL objects relevant to the client link. Every URL object will contain an href and a type field.
+     *
+     * @var \stdClass
+     */
+    public $_links;
+    /**
+     * Get the redirect URL where the customer can complete the payment.
+     *
+     * @return string|null
+     */
+    public function getRedirectUrl(string $client_id, string $state, array $scopes = [], string $approval_prompt = \Mollie\Api\Types\ApprovalPrompt::AUTO)
+    {
+        if (!\in_array($approval_prompt, [\Mollie\Api\Types\ApprovalPrompt::AUTO, \Mollie\Api\Types\ApprovalPrompt::FORCE])) {
+            throw new \Exception('Invalid approval_prompt. Please use "auto" or "force".');
+        }
+        $query = \http_build_query(['client_id' => $client_id, 'state' => $state, 'approval_prompt' => $approval_prompt, 'scope' => \implode(' ', $scopes)], '', '&', \PHP_QUERY_RFC3986);
+        return "{$this->_links->clientLink->href}?{$query}";
+    }
+}