[SPARK-19069][CORE] Expose task 'status' and 'duration' in spark history server REST API.

## What changes were proposed in this pull request?

Although Spark history server UI shows task ‘status’ and ‘duration’ fields, it does not expose these fields in the REST API response. For the Spark history server API users, it is not possible to determine task status and duration. Spark history server has access to task status and duration from event log, but it is not exposing these in API. This patch is proposed to expose task ‘status’ and ‘duration’ fields in Spark history server REST API.

## How was this patch tested?

Modified existing test cases in org.apache.spark.deploy.history.HistoryServerSuite.

Author: Parag Chaudhari <paragpc@amazon.com>

Closes #16473 from paragpc/expose_task_status.