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ARROW-9900: [Rust][DataFusion] Switch from Box -> Arc in LogicalPlanNode

The idea is to continue to simplify the code and improve performance: the inputs to nodes are often copied and using Box requires unnecessary deep copies

I view this as another step towards having LogicalPlan's be made up of `Arc<dyn LogicalPlanNode>` to mirror how `ExecutionPlan`s work

Closes #8098 from alamb/alamb/ARROW-9900-cleanup-box

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Andy Grove <andygrove73@gmail.com>