Provide a utility method to convert client layout and model to server tree and props

keeping assigned to myself. I am actively working on this

The changes under /src were already reviewed by @wim leers in 📌 Provide a utility method to convert client layout and model to server tree and props Active . the the new function \Drupal\experience_builder\Controller\ClientServerConversionTrait::convertClientToServer is mostly just extracted from \Drupal\experience_builder\ClientDataToEntityConverter::convert.

I added a test for the new function \Drupal\experience_builder\Controller\ClientServerConversionTrait::convertClientToServer but this also covered indirectly because it is used ClientDataToEntityConverter::convert which is gets invoked in ApiPublishAllController and ApiContentUpdateForDemoController so it is covered in their tests.

RTBC'ing based of @f.mazeikis approval

Nice to see a subset land first! 👍👏