Add DcgTruncationLevel to Ranking AutoML API

[jwood803]

When performing Ranking in AutoML, it will always return back a length of 3 in the DCG and nDCG metrics. However, when using the Evaluate method for Ranking, it has a property in the RankingEvaluatorOptions to specify the DcgTruncationLevel.

var rankingEvaluatorOptions = new RankingEvaluatorOptions { DcgTruncationLevel = 10 };

This was noticed when adding the Ranking AutoML sample in this comment