As discussed in #31 with the current version of mealie the step-images are missing. That is due the following behavior of mealie and kptncook:

1. kptncook creates recipe in mealie and uploads assets (working fine)
2. kptncook uploades recipe-json
3. mealie expects an assets-node. That was already implemented in kptncook but it was never filled, so an empty assets-node was sent to mealie
4. mealie deletes all assets that are not in the assets node from the uploaded json --> all our uploaded assets are getting deleted again

This PR fills the assets node with the metadata of the uploaded assets.