## What changes were proposed in this pull request?

`mapGroupsWithState` is a new API for arbitrary stateful operations in Structured Streaming, similar to `DStream.mapWithState`

*Requirements*
- Users should be able to specify a function that can do the following
- Access the input row corresponding to a key
- Access the previous state corresponding to a key
- Optionally, update or remove the state
- Output any number of new rows (or none at all)

*Proposed API*
```
// ------------ New methods on KeyValueGroupedDataset ------------
class KeyValueGroupedDataset[K, V] {
	// Scala friendly
	def mapGroupsWithState[S: Encoder, U: Encoder](func: (K, Iterator[V], KeyedState[S]) => U)
        def flatMapGroupsWithState[S: Encode, U: Encoder](func: (K, Iterator[V], KeyedState[S]) => Iterator[U])
	// Java friendly
       def mapGroupsWithState[S, U](func: MapGroupsWithStateFunction[K, V, S, R], stateEncoder: Encoder[S], resultEncoder: Encoder[U])
       def flatMapGroupsWithState[S, U](func: FlatMapGroupsWithStateFunction[K, V, S, R], stateEncoder: Encoder[S], resultEncoder: Encoder[U])
}

// ------------------- New Java-friendly function classes -------------------
public interface MapGroupsWithStateFunction<K, V, S, R> extends Serializable {
  R call(K key, Iterator<V> values, state: KeyedState<S>) throws Exception;
}
public interface FlatMapGroupsWithStateFunction<K, V, S, R> extends Serializable {
  Iterator<R> call(K key, Iterator<V> values, state: KeyedState<S>) throws Exception;
}

// ---------------------- Wrapper class for state data ----------------------
trait State[S] {
	def exists(): Boolean
  	def get(): S 			// throws Exception is state does not exist
	def getOption(): Option[S]
	def update(newState: S): Unit
	def remove(): Unit		// exists() will be false after this
}
```

Key Semantics of the State class
- The state can be null.
- If the state.remove() is called, then state.exists() will return false, and getOption will returm None.
- After that state.update(newState) is called, then state.exists() will return true, and getOption will return Some(...).
- None of the operations are thread-safe. This is to avoid memory barriers.

*Usage*
```
val stateFunc = (word: String, words: Iterator[String, runningCount: KeyedState[Long]) => {
    val newCount = words.size + runningCount.getOption.getOrElse(0L)
    runningCount.update(newCount)
   (word, newCount)
}

dataset					                        // type is Dataset[String]
  .groupByKey[String](w => w)        	                // generates KeyValueGroupedDataset[String, String]
  .mapGroupsWithState[Long, (String, Long)](stateFunc)	// returns Dataset[(String, Long)]
```

## How was this patch tested?
New unit tests.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #16758 from tdas/mapWithState.