[SPARK-16398][CORE] Make cancelJob and cancelStage APIs public

## What changes were proposed in this pull request?

Make SparkContext `cancelJob` and `cancelStage` APIs public. This allows applications to use `SparkListener` to do their own management of jobs via events, but without using the REST API.

## How was this patch tested?

Existing tests (dev/run-tests)

Author: MasterDDT <miteshp@live.com>

Closes #14072 from MasterDDT/SPARK-16398.

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

@@ -2011,13 +2011,23 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
     dagScheduler.cancelAllJobs()
   }
 
-  /** Cancel a given job if it's scheduled or running */
-  private[spark] def cancelJob(jobId: Int) {
+  /**
+   * Cancel a given job if it's scheduled or running.
+   *
+   * @param jobId the job ID to cancel
+   * @throws InterruptedException if the cancel message cannot be sent
+   */
+  def cancelJob(jobId: Int) {
     dagScheduler.cancelJob(jobId)
   }
 
-  /** Cancel a given stage and all jobs associated with it */
-  private[spark] def cancelStage(stageId: Int) {
+  /**
+   * Cancel a given stage and all jobs associated with it.
+   *
+   * @param stageId the stage ID to cancel
+   * @throws InterruptedException if the cancel message cannot be sent
+   */
+  def cancelStage(stageId: Int) {
     dagScheduler.cancelStage(stageId)
   }