Expose `LogicalPlan.to_variant()` as public and expand missing arm handles

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
`LogicalPlan.to_variant()` is currently private and cannot be accessed by a third party library. This is a safe function to mark as public and should be so. Also there are some `todo!()` arms that can now be filled with real logic.
