Add Refunds class and improve php 7.3 coverage

Changelog

[x] Implement the refunds method on https://developers.xendit.co/api-reference/#refunds
[x] Update typed declarations for much strict data passed into method params, ref https://www.php.net/manual/en/language.types.declarations.php
[x] Update old method and replace array_push($args,'value'); with $args[] = 'value';
[x] Update DisbursementPHPTest, DisbursementChannelsTest error catching from \Xendit\Exceptions\ApiException into TypeError exception. This changes to accomodate typed declarations provided by php itself
[x] Avoid deprecations notice by upgrade actions/cache from 2 into 3.2.6. Ref https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

cc @xen-HendryZheng