The base extension class automatically creates a Configuration instance
when a Configuration class exists in the namespace of the extension. But
PHPStan obviously doesn't understand this behaviour and always assumes
that `getConfiguration()` returns `ConfigurationInterface|null` meaning
that the default pattern to get and parse the configuration reports an
error.
I.e.:
```php
namespace Foo;
class SomeExtension extends Extension
{
 public function load(array $configs, ContainerBuilder $container): void
 {
 $configuration = $this->getConfiguration($configs, $container);
 $config = $this->processConfiguration($configuration, $configs);
 }
}
```
results in an error because `processConfiguration()` doesn't accept
`ConfigurationInterface|null`. But when a `Configuration` class exists
in the same namespace as the `Extension` class (so `Foo\Extension`) an
instance of it is returned.
This `DynamicReturnTypeExtension` overrides the return type of
`Extension::getConfiguration()` so it automatically narrows the return
type in case `getConfiguration()` is not overriden and a `Configuration`
class exists. So that in the given example `getConfiguration()` doesn't
return `ConfigurationInterface|null` anymore but `Foo\Configuration` and
there is no error on calling `processConfiguration()`.