[SPARK-37497][K8S] Promote ExecutorPods[PollingSnapshot|WatchSnapshot]Source to DeveloperApi

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

This PR aims to promote `ExecutorPodsWatchSnapshotSource` and `ExecutorPodsPollingSnapshotSource` as **stable** `DeveloperApi` in order to maintain it officially in a backward compatible way at Apache Spark 3.3.0.

### Why are the changes needed?

- Since SPARK-24248 at Apache Spark 2.4.0, `ExecutorPodsWatchSnapshotSource` and `ExecutorPodsPollingSnapshotSource` have been used to monitor executor pods without any interface changes for over 3 years.

- Apache Spark 3.1.1 makes `Kubernetes` module GA and provides an extensible external cluster manager framework. New `ExternalClusterManager` for K8s environment need to depend on this to monitor pods.

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

No.

### How was this patch tested?

Manual review.

Closes apache#34751 from dongjoon-hyun/SPARK-37497.

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

Author: dongjoon-hyun

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/scheduler/cluster/k8s/ExecutorPodsPollingSnapshotSource.scala

@@ -24,12 +24,21 @@ import io.fabric8.kubernetes.client.KubernetesClient
 import scala.collection.JavaConverters._
 
 import org.apache.spark.SparkConf
+import org.apache.spark.annotation.{DeveloperApi, Since, Stable}
 import org.apache.spark.deploy.k8s.Config._
 import org.apache.spark.deploy.k8s.Constants._
 import org.apache.spark.internal.Logging
 import org.apache.spark.util.{ThreadUtils, Utils}
 
-private[spark] class ExecutorPodsPollingSnapshotSource(
+/**
+ * :: DeveloperApi ::
+ *
+ * A class used for polling K8s executor pods by ExternalClusterManagers.
+ * @since 3.1.3
+ */
+@Stable
+@DeveloperApi
+class ExecutorPodsPollingSnapshotSource(
     conf: SparkConf,
     kubernetesClient: KubernetesClient,
     snapshotsStore: ExecutorPodsSnapshotsStore,
@@ -39,13 +48,15 @@ private[spark] class ExecutorPodsPollingSnapshotSource(
 
   private var pollingFuture: Future[_] = _
 
+  @Since("3.1.3")
   def start(applicationId: String): Unit = {
     require(pollingFuture == null, "Cannot start polling more than once.")
     logDebug(s"Starting to check for executor pod state every $pollingInterval ms.")
     pollingFuture = pollingExecutor.scheduleWithFixedDelay(
       new PollRunnable(applicationId), pollingInterval, pollingInterval, TimeUnit.MILLISECONDS)
   }
 
+  @Since("3.1.3")
   def stop(): Unit = {
     if (pollingFuture != null) {
       pollingFuture.cancel(true)

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/scheduler/cluster/k8s/ExecutorPodsWatchSnapshotSource.scala

@@ -22,16 +22,27 @@ import io.fabric8.kubernetes.api.model.Pod
 import io.fabric8.kubernetes.client.{KubernetesClient, Watcher, WatcherException}
 import io.fabric8.kubernetes.client.Watcher.Action
 
+import org.apache.spark.annotation.{DeveloperApi, Since, Stable}
 import org.apache.spark.deploy.k8s.Constants._
 import org.apache.spark.internal.Logging
 import org.apache.spark.util.Utils
 
-private[spark] class ExecutorPodsWatchSnapshotSource(
+/**
+ * :: DeveloperApi ::
+ *
+ * A class used for watching K8s executor pods by ExternalClusterManagers.
+ *
+ * @since 3.1.3
+ */
+@Stable
+@DeveloperApi
+class ExecutorPodsWatchSnapshotSource(
     snapshotsStore: ExecutorPodsSnapshotsStore,
     kubernetesClient: KubernetesClient) extends Logging {
 
   private var watchConnection: Closeable = _
 
+  @Since("3.1.3")
   def start(applicationId: String): Unit = {
     require(watchConnection == null, "Cannot start the watcher twice.")
     logDebug(s"Starting watch for pods with labels $SPARK_APP_ID_LABEL=$applicationId," +
@@ -42,6 +53,7 @@ private[spark] class ExecutorPodsWatchSnapshotSource(
       .watch(new ExecutorPodsWatcher())
   }
 
+  @Since("3.1.3")
   def stop(): Unit = {
     if (watchConnection != null) {
       Utils.tryLogNonFatalError {