Enhance kind of values accepted in $className of ExistValidator and UniqueValidator

Usage sample:

```php
namespace frontend\models;

class User extends \common\models\User
{
    public function rules()
    {
        return [
            ['group_id', 'unique', 'className' => 'Group',
                'attributeName' => 'id'],
            
            ['region_id', 'unique', 'className' => '\common\models\Region',
                'attributeName' => 'id'],
                
            ['username', 'unique'],
        ];
    }
}

class Group extends \common\models\Group
{
}

namespace common\models;

class Region extends \yii\db\ActiveRecord
{
}
```