Add map_children_mut to Expr / maybe TreeNode? #22399

#22298 added an optimization for LogicalPlan rewriting to avoid cloning the tree when not necessary via a new map_children_mut method.

We could probably add something similar to Expr. I'm less sure about Arc<dyn PhysicalExpr> and Arc<dyn ExecutionPlan> but we should generally explore our options for this method or other changes we might be able to make to the TreeNode API to make optimizer rules more efficient.