## What changes were proposed in this pull request?
This patch annotates the InterfaceStability level for top level classes in o.a.spark.sql and o.a.spark.sql.util packages, to experiment with this new annotation.

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #15392 from rxin/SPARK-17830.