[Python] Expose grouping segment keys to PyArrow

This is a [follow-up task](https://github.com/apache/arrow/pull/14352#discussion_r1026926422) for a PR.

**Reporter**: [Yaron Gvili](https://issues.apache.org/jira/browse/ARROW-18368) / @rtpsw

_{**Note**: *This issue was originally created as [ARROW-18368](https://issues.apache.org/jira/browse/ARROW-18368). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the linked PR discussion, since the issue provides no file, test, or entry point. Determine which PyArrow grouping API should expose the segment keys and define completion as those keys being available through PyArrow with appropriate coverage.

Written by the indexing model from the issue text.