API login broken for non-ASCII usernames

[samuelvogel]

We've had multiple occurrences of users not being able to log in via the API, because their username contained non-ASCII characters. See the following Shopware JIRA Issue as well: https://issues.shopware.com/issues/SW-15358

Unfortunately this is deeply buried inside the Zend Framework 1 which is shipped with Shopware: https://github.com/zendframework/zf1/blob/a90f3a8d71e0788020f730da83674b7312bd3b16/library/Zend/Auth/Adapter/Http.php#L527-L532

The ctype_print function is used to check the base64-decoded Authorization header for non-ASCII characters. See here fore the referenced ticket ZF-1253: https://framework.zend.com/issues/browse/ZF-1253

As there is no realistic way to fix this, we should implement a special validation in the Shopware backend user administration which only allows ASCII characters in the username when the API key (and therefore API access) is activated: