[SPARK-14516][ML] Adding ClusteringEvaluator with the implementation of Cosine silhouette and squared Euclidean silhouette.

## What changes were proposed in this pull request?

This PR adds the ClusteringEvaluator Evaluator which contains two metrics:
 - **cosineSilhouette**: the Silhouette measure using the cosine distance;
 - **squaredSilhouette**: the Silhouette measure using the squared Euclidean distance.

The implementation of the two metrics refers to the algorithm proposed and explained [here](https://drive.google.com/file/d/0B0Hyo%5f%5fbG%5f3fdkNvSVNYX2E3ZU0/view). These algorithms have been thought for a distributed and parallel environment, thus they have reasonable performance, unlike a naive Silhouette implementation following its definition.

## How was this patch tested?

The patch has been tested with the additional unit tests added (comparing the results with the ones provided by [Python sklearn library](http://scikit-learn.org/stable/modules/generated/sklearn.metrics.silhouette_score.html)).

Author: Marco Gaido <[email protected]>

Closes apache#18538 from mgaido91/SPARK-14516.

Author: mgaido91