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ARROW-9833: [Rust] [DataFusion] TableProvider.scan now returns ExecutionPlan

`TableProvider.scan()` now returns `ExecutionPlan` instead of `Vec<Partition>`.

This is a step towards removing the `Partition` trait and passing the `partition_id` to `ExecutionPlan.execute()` instead.

I also made some minor improvements to the unit tests to reduce duplicate code and use `?` instead of `unwrap()`.

Closes #8028 from andygrove/ARROW-9833

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