Stop copying LogicalPlan and Exprs in EliminateNestedUnion #10296

Is your feature request related to a problem or challenge?

Part of #9637

As part of making the planner faster, we are updating the optimizer passes to avoid copying LogicalPlan and Expr (see #9637)

Describe the solution you'd like

I would like to reduce the amount of copying in this pass (even though it doesn't appear in current profiling)

Describe alternatives you've considered

Apply the model from @Lordworms in #10166 to this pass
2. Update OptimizerRule::supports_rewrite` to return true

1. Update OptimizerRule to use rewrite
2. Update the pass itself to not copy the LogicalPlan (ideally using the TreeNode API) - it is implemented for LogicalPlan (API) and Expr (API)

Other examples: #10218

Additional context

I think this will be a pretty simple refactor with a good existing pattern, so mark this as good first issue