Proxy services require now manual work

The previous proxy class implementation had several issues, like for example it required all interfaces to be loaded on every request.
The solution was to use just actual PHP code, which is generated by a dump script.

On top of flagging your service as lazy: true now, you need on top
generate those proxy classes.

Here is an example how to call it:

php core/scripts/generate-proxy-class.php "Drupal\views_ui\ParamConverter\ViewUIConverter" core/modules/views_ui/src

This class needs to be added to your version control system of choice and that's it