[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.
Instead of setting up and using testOperationWithInitial just for this unit test, lets try to do the alternative. 
```
val updateStateOperation = (s: DStream[String]) => {
      val initialRDD = s.context.sparkContext.makeRDD(initial)
      val updateFunc = (values: Seq[Int], state: Option[Int]) => {
        Some(values.sum + state.getOrElse(0))
      }
      val newUpdateFunc = (iterator: Iterator[(String, Seq[Int], Option[Int])]) => {
        iterator.flatMap(t => updateFunc(t._2, t._3).map(s => (t._1, s)))
      }
      s.map(x => (x, 1)).updateStateByKey[Int](newUpdateFunc,
        new HashPartitioner (numInputPartitions), true, initialRDD)
    }
```

And then use the usual `testOperation`