[SPARK-14862][ML] Updated Classifiers to not require labelCol metadata

## What changes were proposed in this pull request?

Updated Classifier, DecisionTreeClassifier, RandomForestClassifier, GBTClassifier to not require input column metadata.
* They first check for metadata.
* If numClasses is not specified in metadata, they identify the largest label value (up to a limit).

This functionality is implemented in a new Classifier.getNumClasses method.

Also
* Updated Classifier.extractLabeledPoints to (a) check label values and (b) include a second version which takes a numClasses value for validity checking.

## How was this patch tested?

* Unit tests in ClassifierSuite for helper methods
* Unit tests for DecisionTreeClassifier, RandomForestClassifier, GBTClassifier with toy datasets lacking label metadata

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12663 from jkbradley/trees-no-metadata.