Support 8.0 attributes in extension stubs

### Description

Hello!

We'd like to use 8.0 attributes in generated arginfo files for our extensions.

it seems that `#[\SensitiveParameter]` is supported, but that's about not:

```php
namespace MyExtension {
​
  class Foobar {
    
    #[\MyExtension\FooAttribute]
    public function getBaz() {
      //
    }
​
  }

  class FooAttribute { }
​
}
```