[SPARK-15452][SQL] Mark aggregator API as experimental

## What changes were proposed in this pull request?
The Aggregator API was introduced in 2.0 for Dataset. All typed Dataset APIs should still be marked as experimental in 2.0.

## How was this patch tested?
N/A - annotation only change.

Author: Reynold Xin <rxin@databricks.com>

Closes #13226 from rxin/SPARK-15452.