## What changes were proposed in this pull request?

Currently we can't produce a `Dataset` containing `Set` in SparkSQL. This PR tries to support serialization/deserialization of `Set`.

Because there's no corresponding internal data type in SparkSQL for a `Set`, the most proper choice for serializing a set should be an array.

## How was this patch tested?

Added unit tests.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #18416 from viirya/SPARK-21204.