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/Balance.php

@@ -0,0 +1,101 @@
+<?php
+
+namespace Mollie\Api\Resources;
+
+class Balance extends \Mollie\Api\Resources\BaseResource
+{
+    /**
+     * Indicates this is a balance resource. The value will always be "balance" here.
+     *
+     * @var string
+     */
+    public $resource;
+    /**
+     * The mode used to create this balance. Mode determines whether real or test payments can be moved to this balance.
+     * The value is either "live" or "test".
+     *
+     * @var string
+     */
+    public $mode;
+    /**
+     * The identifier uniquely referring this balance. Mollie assigns this identifier at balance creation.
+     *
+     * @example bal_gVMhHKqSSRYJyPsuoPABC
+     * @var string
+     */
+    public $id;
+    /**
+     * UTC datetime the balance was created in ISO-8601 format.
+     *
+     * @example "2021-12-25T10:30:54+00:00"
+     * @var string
+     */
+    public $createdAt;
+    /**
+     * The balance's ISO 4217 currency code.
+     *
+     * @var string
+     */
+    public $currency;
+    /**
+     * The status of the balance: "active" if the balance is operational and ready to be used.
+     * The status is "inactive" if the account is still being validated by Mollie or if the balance has been blocked.
+     *
+     * @var string
+     */
+    public $status;
+    /**
+     * The total amount directly available on the balance.
+     *
+     * @var \stdClass
+     */
+    public $availableAmount;
+    /**
+     * The total amount queued to be transferred to your balance.
+     * For example, a credit card payment can take a few days to clear.
+     *
+     * @var \stdClass
+     */
+    public $incomingAmount;
+    /**
+     * The total amount that is in the process of being transferred from your balance to your verified bank account.
+     * @var \stdClass
+     */
+    public $outgoingAmount;
+    /**
+     * The frequency at which the available amount on the balance will be transferred away to the configured transfer
+     * destination. See "transferDestination". Note that if the transfer is for an external destination, and the
+     * transfer is created in a weekend or during a bank holiday, the actual bank transfer will take place on the next
+     * business day.
+     *
+     * @var string
+     */
+    public $transferFrequency;
+    /**
+     * The minimum amount configured for scheduled automatic balance transfers. As soon as the amount on the balance
+     * exceeds this threshold, the complete balance will be paid out to the "transferDestination" according to the
+     * configured "transferFrequency".
+     *
+     * @var \stdClass
+     */
+    public $transferThreshold;
+    /**
+     * The reference to be included on all transfers for this balance.
+     *
+     * @var string|null
+     */
+    public $transferReference;
+    /**
+     * The destination where the available amount will be automatically transferred to according to the configured
+     * "transferFrequency".
+     *
+     * @var \stdClass
+     */
+    public $transferDestination;
+    /**
+     * Links to help navigate through the Mollie API and related resources.
+     *
+     * @var \stdClass
+     */
+    public $_links;
+}