Fixable compatibility issues with PHP 8.2-8.3

Hi,

I'm using v0.0.47 of the SDK but the following seems to be an issue also on the latest version. Some classes seem to assign properties dynamically without declaring them. This has been deprecated in PHP 8.2 (resulting in thrown deprecation notices) and will result in fatal errors in future versions.

https://www.php.net/manual/en/language.oop5.properties.php#language.oop5.properties.dynamic-properties

Here's a partial list of notices I've seen:

• vendor/cybersource/rest-client-php/lib/Authentication/Core/MerchantConfiguration.php on line 228
• vendor/cybersource/rest-client-php/lib/ApiClient.php on line 108
• vendor/cybersource/rest-client-php/lib/ApiClient.php on line 240
• vendor/cybersource/rest-client-php/lib/Authentication/PayloadDigest/PayloadDigest.php on line 47
• vendor/cybersource/rest-client-php/lib/Authentication/Http/HttpSignatureGenerator.php on line 78
• vendor/cybersource/rest-client-php/lib/Authentication/PayloadDigest/PayloadDigest.php on line 47

there might be more

A quick workaround for this could be to add a #[AllowDynamicProperties] attribute at the start of the affected classes until a more robust handling is in place.

https://www.php.net/manual/en/class.allowdynamicproperties.php

Start by reviewing the listed classes in lib/Authentication/Core/MerchantConfiguration.php, lib/ApiClient.php, lib/Authentication/PayloadDigest/PayloadDigest.php, and lib/Authentication/Http/HttpSignatureGenerator.php, then check for additional affected classes. Reproduce the compatibility notices on PHP 8.2-8.3 and determine whether the affected properties are declared or require the proposed compatibility handling. Done means the SDK no longer emits these dynamic-property deprecation notices.