Fix PHP 8.4 deprecation: use explicit nullable types

[bnzo]

Fixes #42

Summary

PHP 8.4 deprecated implicitly marking typed parameters as nullable via a null default value (e.g. Type $param = null). This triggers E_DEPRECATED warnings at runtime.

This PR updates all 20 occurrences across 15 files to use the explicit nullable type syntax (?Type $param = null), which is supported since PHP 7.1 and is fully backwards compatible.

Files changed

• Client.php — constructor $httpClient
• Api/ApiWrapper.php — $headers param in get(), post(), patch(), delete()
• Api/Auth/Token.php — $code in generate()
• Api/Ais/Connect.php — $scope in generate()
• Api/Customers/Customers.php — $customerId in get()
• Api/Pis/Initiate.php — $state in generate()
• Api/Pis/Payment.php — $sessionId, $withBeneficiary, $withVirtualBeneficiary in get()
• Api/Pis/Refund.php — $state in generate()
• Api/Pis/RequestToPay.php — $redirectUri, $state in generate()
• Api/Pis/Settlement.php — $settlementId in get()
• Api/Resources/Provider.php — $id in get()
• Config/Endpoint.php — $environment in getApiUrl()
• Config/Telemetry.php — $additionalMetrics in logAction() and getMetrics()
• Util/Header.php — $customCredentials in generate()
• Util/PemManager.php — $encryptionManager in constructor

Test plan

• No behavioral changes — only type declaration syntax updated
• All existing tests should pass without modification
• Eliminates all E_DEPRECATED warnings on PHP 8.4+