[SPARK-48077][K8S] Promote KubernetesClientUtils to DeveloperApi

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

This PR aims to promote `KubernetesClientUtils` to `DeveloperApi`.

### Why are the changes needed?

Since `Apache Spark Kubernetes Operator` requires this, we had better maintain it as a developer API officially from `Apache Spark 4.0.0`.
- apache/spark-kubernetes-operator#10

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

No.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #46327 from dongjoon-hyun/SPARK-48077.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

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

@@ -28,23 +28,34 @@ import scala.jdk.CollectionConverters._
 import io.fabric8.kubernetes.api.model.{ConfigMap, ConfigMapBuilder, KeyToPath}
 
 import org.apache.spark.SparkConf
+import org.apache.spark.annotation.{DeveloperApi, Since, Unstable}
 import org.apache.spark.deploy.k8s.{Config, Constants, KubernetesUtils}
 import org.apache.spark.deploy.k8s.Config.{KUBERNETES_DNS_SUBDOMAIN_NAME_MAX_LENGTH, KUBERNETES_NAMESPACE}
 import org.apache.spark.deploy.k8s.Constants.ENV_SPARK_CONF_DIR
 import org.apache.spark.internal.{Logging, MDC}
 import org.apache.spark.internal.LogKeys.{CONFIG, PATH, PATHS}
 import org.apache.spark.util.ArrayImplicits._
 
-private[spark] object KubernetesClientUtils extends Logging {
+/**
+ * :: DeveloperApi ::
+ *
+ * A utility class used for K8s operations internally and Spark K8s operator.
+ */
+@Unstable
+@DeveloperApi
+object KubernetesClientUtils extends Logging {
 
   // Config map name can be KUBERNETES_DNS_SUBDOMAIN_NAME_MAX_LENGTH chars at max.
+  @Since("3.3.0")
   def configMapName(prefix: String): String = {
     val suffix = "-conf-map"
     s"${prefix.take(KUBERNETES_DNS_SUBDOMAIN_NAME_MAX_LENGTH - suffix.length)}$suffix"
   }
 
+  @Since("3.1.0")
   val configMapNameExecutor: String = configMapName(s"spark-exec-${KubernetesUtils.uniqueID()}")
 
+  @Since("3.1.0")
   val configMapNameDriver: String = configMapName(s"spark-drv-${KubernetesUtils.uniqueID()}")
 
   private def buildStringFromPropertiesMap(configMapName: String,
@@ -62,6 +73,7 @@ private[spark] object KubernetesClientUtils extends Logging {
   /**
    * Build, file -> 'file's content' map of all the selected files in SPARK_CONF_DIR.
    */
+  @Since("3.1.1")
   def buildSparkConfDirFilesMap(
       configMapName: String,
       sparkConf: SparkConf,
@@ -77,6 +89,7 @@ private[spark] object KubernetesClientUtils extends Logging {
     }
   }
 
+  @Since("3.1.0")
   def buildKeyToPathObjects(confFilesMap: Map[String, String]): Seq[KeyToPath] = {
     confFilesMap.map {
       case (fileName: String, _: String) =>
@@ -89,6 +102,7 @@ private[spark] object KubernetesClientUtils extends Logging {
    * Build a Config Map that will hold the content for environment variable SPARK_CONF_DIR
    * on remote pods.
    */
+  @Since("3.1.0")
   def buildConfigMap(configMapName: String, confFileMap: Map[String, String],
       withLabels: Map[String, String] = Map()): ConfigMap = {
     val configMapNameSpace =