arrow

ARROW-9464: [Rust] [DataFusion] Remove Partition trait

This follows on from https://github.com/apache/arrow/pull/8028.

- Removes `Partition` trait, which was really redundant.
- `ExecutonPlan.execute()` now takes a partition index.
- Introduced `Partitioning` enum so that execution plans can describe their partitioning scheme and number of partitions. Currently, it just has `UnknownPartitioning` but later we can add others, such as `HashPartitioning`.
- Removed `DatasourceExec` since no longer needed now that `TableProvider.scan` returns an `ExecutionPlan` directly.

This is a step towards extracting the threading model out of the operators like `MergeExec` and having the operators be able to work with different threading models.

Closes #8029 from andygrove/execute-partition

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