allow usage of string in getAttributeValueObject

I've noticed that most people use $c->getAttribute to display an attribute value. This is imho not always the
best solution, lots of attribute types contain a nice getDisplayValue method which will be ignored because of that.

I think this code would be better:

```php
$ak = CollectionAttributeKey::getByHandle('test');
$valueObject = $p->getAttributeValueObject($ak);
echo $valueObject->getValue('display');
```

This is of course much longer than a simple getAttribute. That's why I thought we could check for a string value
which would make this possible:
```php
$valueObject = $p->getAttributeValueObject('test');
echo $valueObject->getValue('display');
```

Still a bit longer but already a bit shorter