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ARROW-9758: [Rust] [DataFusion] Allow physical planner to be replaced

This PR mainly moves the physical query planning logic out of ExecutionContext and into its own struct. There is a new `PhysicalPlanner` trait, and it is now possible to bring-your-own planner if the one in DataFusion doesn't meet your needs (for example, if you need to implement distributed query execution).

pseudo code example:

```rust
let config = ExectionConfig::new().with_physical_planner(Arc::new(MyPhysicalPlanner::new()));
let ctx = ExecutionContext::new(config);
ctx.sql("SELECT * FROM foo");
```

Closes #7975 from andygrove/physical-planner-trait

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