Major Description
With the DataSourceV2 write operations, we have a way to fallback to the V1 writer APIs using InsertableRelation.
The gross part is that we're in physical land, but the InsertableRelation takes a logical plan, so we have to pass the logical plans to these physical nodes, and then potentially go through re-planning.
A useful primitive could be specifying that a plan is ready for execution through a logical node AlreadyPlanned. This would wrap a physical plan, and then we can go straight to execution.
