[SPARK-3660][STREAMING] Initial RDD for updateStateByKey transformation

SPARK-3660 : Initial RDD for updateStateByKey transformation

I have added a sample StatefulNetworkWordCountWithInitial inspired by StatefulNetworkWordCount.

Please let me know if any changes are required.
I guess this makes it different from the Scala api, and the reason is that the Java API does not expose the version of the API that uses iterators. And as far as I remember, that was not done because its tricky to write a function Java dealing with scala iterators and all. 

Lets try to make the Java and Scala API consistent (minus the iterator based ones). So could you please add a Scala API equivalent to this, that is, (simple non-iterator function, partitioner, initial RDD)? Also unit test for that please :)