[SPARK-33724][K8S] Add decom script as a configuration param

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

Makes the location of the decommission script used in Kubernetes for graceful shutdown configurable.

### Why are the changes needed?

Some environments don't use the Spark image builder and instead mount the decompressed Spark distro. In those envs configuring the location of the decommissioning script is required.

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

New configuration parameter.

### How was this patch tested?

Existing decommissioning integration test.

Closes #30694 from holdenk/SPARK-33724-allow-decommissioning-script-location-to-be-configured.

Authored-by: Holden Karau <hkarau@apple.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/Config.scala

@@ -24,6 +24,14 @@ import org.apache.spark.internal.config.ConfigBuilder
 
 private[spark] object Config extends Logging {
 
+  val DECOMMISSION_SCRIPT =
+    ConfigBuilder("spark.kubernetes.decommission.script")
+      .doc("The location of the script to use for graceful decommissioning")
+      .version("3.2.0")
+      .stringConf
+      .createWithDefault("/opt/decom.sh")
+
+
   val KUBERNETES_CONTEXT =
     ConfigBuilder("spark.kubernetes.context")
       .doc("The desired context from your K8S config file used to configure the K8S " +

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/features/BasicExecutorFeatureStep.scala

@@ -230,7 +230,7 @@ private[spark] class BasicExecutorFeatureStep(
         new ContainerBuilder(containerWithLimitCores).withNewLifecycle()
           .withNewPreStop()
             .withNewExec()
-              .addToCommand("/opt/decom.sh")
+              .addToCommand(kubernetesConf.get(DECOMMISSION_SCRIPT))
             .endExec()
           .endPreStop()
           .endLifecycle()