[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.