Allow overriding the API credentials in settings.php

While tracking through the code I found it isn't really easy to do, so I thought adding a drupal_alter thing would be useful.

Hm... We probably could find a better way to do this... Wondering if we need a broader initiative that allows overridding the configuration of any shipping method via the Key module or something.
The current patch is adding direct \Drupal calls() which is not ideal and the suggested approach is really USPS specific.