## What changes were proposed in this pull request?

This PR adds `pivot` function to SparkR for API parity. Since this PR is based on https://github.com/apache/spark/pull/13295 , mhnatiuk should be credited for the work he did.

## How was this patch tested?

Pass the Jenkins tests (including new testcase.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13786 from dongjoon-hyun/SPARK-15294.