[SPARK-20861][ML][PYTHON] Delegate looping over paramMaps to estimators

Changes:

pyspark.ml Estimators can take either a list of param maps or a dict of params. This change allows the CrossValidator and TrainValidationSplit Estimators to pass through lists of param maps to the underlying estimators so that those estimators can handle parallelization when appropriate (eg distributed hyper parameter tuning).

Testing:

Existing unit tests.

Author: Bago Amirbekian <bago@databricks.com>

Closes #18077 from MrBago/delegate_params.