Calibration estimators in ML.NET

[abgoswam]

In the discussions for #1579, one of the proposals was to introduce calibration estimators in ML.NET.

(from #1579)

.... a calibrator is just one peculiar form of trainer: it learns a monotonous function that transforms 'scores' into 'probabilities', with the goal to minimize the log-loss against the 'target label'. So, it is actually a univariate classification trainer. We should create a PlattCalibrationEstimator to train Platt calibrators and a PavCalibrationEstimator to train PAV calibrators.

@Zruty0 @yaeldekel