I would suggest that you add your function and libraries in the event/custom code that you create. For example, suppose that you need to include some google drive library and some new functions to the save event of a document. You decide to add a vtiger.aftersave event. In the file where your new functionality resides, you will add the library include at the beginning and your functions inside the new class so you don't need to modify the common utils base code. Something like this:

<?php
require_once('google drive library');
 
class YourCustomEventHandlerClass extends VTEventHandler {
 
    function YourNewFunction() {
     ...
    }
 
    function handleEvent($eventName, $entityData) {
      global $log, $adb;
      ...
      $this->YourNewFunction();
      ...
    }
}
?>