Add DSL functions invoking percentile_approx

Since 2.1 (SPARK-16283) Spark provides percentile_approx SQL function, however it is not exposed through DSL, with exception to `DataFrameStatFunctions.approxQuantile` (which is eager and doesn't support grouping).

It would be useful to have it exposed through o.a.s.sql.functions / pyspark.sql.functions.