[SPARK-16074][MLLIB] expose VectorUDT/MatrixUDT in a public API

## What changes were proposed in this pull request?

Both VectorUDT and MatrixUDT are private APIs, because UserDefinedType itself is private in Spark. However, in order to let developers implement their own transformers and estimators, we should expose both types in a public API to simply the implementation of transformSchema, transform, etc. Otherwise, they need to get the data types using reflection.

## How was this patch tested?

Unit tests in Scala and Java.

Author: Xiangrui Meng <meng@databricks.com>

Closes #13789 from mengxr/SPARK-16074.