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ARROW-7941: [Rust] [DataFusion] Add support for named columns in logical plan

This PR adds support for unresolved columns in the logical plan so that users can add columns by name rather than index. There is a new optimizer rule that will resolve these columns and replace them with indices in the plan.

This PR also:

- Removes pointless `Arc`s from the optimizer rules
- Optimizer rules now leverage `LogicalPlanBuilder` for much more concise and readable code

Closes #6730 from andygrove/ARROW-7941

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>