[SPARK-15483][SQL] IncrementalExecution should use extra strategies.

## What changes were proposed in this pull request?

Extra strategies does not work for streams because `IncrementalExecution` uses modified planner with stateful operations but it does not include extra strategies.

This pr fixes `IncrementalExecution` to include extra strategies to use them.

## How was this patch tested?

I added a test to check if extra strategies work for streams.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #13261 from ueshin/issues/SPARK-15483.

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/IncrementalExecution.scala

@@ -36,7 +36,8 @@ class IncrementalExecution private[sql](
   extends QueryExecution(sparkSession, logicalPlan) {
 
   // TODO: make this always part of planning.
-  val stateStrategy = sparkSession.sessionState.planner.StatefulAggregationStrategy :: Nil
+  val stateStrategy = sparkSession.sessionState.planner.StatefulAggregationStrategy +:
+    sparkSession.sessionState.experimentalMethods.extraStrategies
 
   // Modified planner with stateful operations.
   override def planner: SparkPlanner =

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala

@@ -220,6 +220,21 @@ class StreamSuite extends StreamTest with SharedSQLContext {
       CheckOffsetLogLatestBatchId(2),
       CheckSinkLatestBatchId(2))
   }
+
+  test("insert an extraStrategy") {
+    try {
+      spark.experimental.extraStrategies = TestStrategy :: Nil
+
+      val inputData = MemoryStream[(String, Int)]
+      val df = inputData.toDS().map(_._1).toDF("a")
+
+      testStream(df)(
+        AddData(inputData, ("so slow", 1)),
+        CheckAnswer("so fast"))
+    } finally {
+      spark.experimental.extraStrategies = Nil
+    }
+  }
 }
 
 /**