[SPARK-42357][CORE] Log exitCode when SparkContext.stop starts

### What changes were proposed in this pull request?

This PR aims to log `exitCode` when `SparkContext.stop` starts as a clear boundary to ignore the meaningless log messages from user jobs.

### Why are the changes needed?

This PR adds the following log.
```
23/02/06 02:12:55 INFO SparkContext: SparkContext is stopping with exitCode 0.
```

In the simplest case, it stops like the following.

```
$ bin/spark-submit examples/src/main/python/pi.py
...
Pi is roughly 3.147080
23/02/06 02:12:55 INFO SparkContext: SparkContext is stopping with exitCode 0.
23/02/06 02:12:55 INFO AbstractConnector: Stopped Spark1cb72b8{HTTP/1.1, (http/1.1)}{localhost:4040}
23/02/06 02:12:55 INFO SparkUI: Stopped Spark web UI at http://localhost:4040
23/02/06 02:12:55 INFO MapOutputTrackerMasterEndpoint: MapOutputTrackerMasterEndpoint stopped!
23/02/06 02:12:55 INFO MemoryStore: MemoryStore cleared
23/02/06 02:12:55 INFO BlockManager: BlockManager stopped
23/02/06 02:12:55 INFO BlockManagerMaster: BlockManagerMaster stopped
23/02/06 02:12:55 INFO OutputCommitCoordinator$OutputCommitCoordinatorEndpoint: OutputCommitCoordinator stopped!
23/02/06 02:12:55 INFO SparkContext: Successfully stopped SparkContext
23/02/06 02:12:56 INFO ShutdownHookManager: Shutdown hook called
```

However, in the complex case, there are many many logs after invoking `SparkContet.stop(0)`. Sometimes, this makes users confused. New log will show a clear boundary to ignore the meaningless messages.
```
23/02/06 02:59:27 INFO TaskSetManager: Starting task 283.0 in stage 34.0 (TID 426) (172.31.218.234, executor 5, partition 283, PROCESS_LOCAL, 8001 bytes)
...
23/02/06 02:59:27 INFO BlockManagerInfo: Removed broadcast_35_piece0 on 172.31.218.244:41741 in memory (size: 5.7 KiB, free: 50.8 GiB)
...
23/02/06 02:59:27 INFO SparkUI: Stopped Spark web UI at http://r6i-16xlarge-3402-0203-apple-3-bf3f7e8624a90a37-driver-svc.default.svc:4040
...
23/02/06 02:59:27 INFO DAGScheduler: ShuffleMapStage 34 (q24a) failed in 0.103 s due to Stage cancelled because SparkContext was shut down
...
23/02/06 02:59:27 INFO KubernetesClusterSchedulerBackend: Shutting down all executors
...
23/02/06 02:59:27 INFO KubernetesClusterSchedulerBackend$KubernetesDriverEndpoint: Asking each executor to shut down
...
23/02/06 02:59:27 ERROR TransportRequestHandler: Error while invoking RpcHandler#receive() for one-way message.
org.apache.spark.SparkException: Could not find CoarseGrainedScheduler.
```

### Does this PR introduce _any_ user-facing change?

No, this is a log-only change.

### How was this patch tested?

Manually.

Closes #39900 from dongjoon-hyun/SPARK-42357.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -2092,6 +2092,7 @@ class SparkContext(config: SparkConf) extends Logging {
    * @param exitCode Specified exit code that will passed to scheduler backend in client mode.
    */
   def stop(exitCode: Int): Unit = {
+    logInfo(s"SparkContext is stopping with exitCode $exitCode.")
     if (LiveListenerBus.withinListenerThread.value) {
       throw new SparkException(s"Cannot stop SparkContext within listener bus thread.")
     }