[SPARK-19636][ML] Feature parity for correlation statistics in MLlib

## What changes were proposed in this pull request?

This patch adds the Dataframes-based support for the correlation statistics found in the `org.apache.spark.mllib.stat.correlation.Statistics`, following the design doc discussed in the JIRA ticket.

The current implementation is a simple wrapper around the `spark.mllib` implementation. Future optimizations can be implemented at a later stage.

## How was this patch tested?

```
build/sbt "testOnly org.apache.spark.ml.stat.StatisticsSuite"
```

Author: Timothy Hunter <timhunter@databricks.com>

Closes #17108 from thunterdb/19636.