Add ability to unpublish assets from asset-library

[whogben]

I discovered an issue with my addons that requires removing them from the asset-library (godotengine/godot-proposals#554 (comment)) but there doesn't seem to be a way to do this.

Ideally, I'd just turn off a checkbox like "Show in Asset Library" or hit a button like "Unpublish."

As a workaround, I submitted an edit and I modified the asset's title and description to request the reviewer to remove it, however this seems hacky - and since it relies on review and has a delay, users may download the non-functioning addon in the meantime - hence why I'm adding the issue as a feature suggestion here.

[Calinou]

Thanks for the report! Consolidating in #157.