[SPARK-16446][SPARKR][ML] Gaussian Mixture Model wrapper in SparkR

## What changes were proposed in this pull request?
Gaussian Mixture Model wrapper in SparkR, similarly to R's ```mvnormalmixEM```.

## How was this patch tested?
Unit test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #14392 from yanboliang/spark-16446.