[SPARK-3486][MLlib][PySpark] PySpark support for Word2Vec

@mengxr
Added PySpark support for Word2Vec
Change list
(1) PySpark support for Word2Vec
(2) SerDe support of string sequence both on python side and JVM side
(3) Test for SerDe of string sequence on JVM side
@davies Are we always using batched serialization? The pythonToJava funciton at PythonRDD returns JavaRDD[Any]. Should I use JavaRDD[java.util.ArrayList[String]] as the return type? Thanks!