[C++] Substrait to Arrow Aggregate doesn't take the provided Output Type for aggregates

The current Substrait to Aggregate deserializer doesn't take the plan provided output type as the output type of the execution plan.

**Reporter**: [Vibhatha Lakmal Abeykoon](https://issues.apache.org/jira/browse/ARROW-17484) / @vibhatha

_{**Note**: *This issue was originally created as [ARROW-17484](https://issues.apache.org/jira/browse/ARROW-17484). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Locate the Substrait-to-Aggregate deserializer and trace how the plan's provided output type is handled when constructing the execution plan. Confirm the aggregate execution plan uses that provided type rather than an inferred type, and add or update coverage for this deserialization path if an existing test entry point is found.

Written by the indexing model from the issue text.