[SPARK-11155][WEB UI] Stage summary json should include stage duration

The json endpoint for stages doesn't include information on the stage duration that is present in the UI. This looks like a simple oversight, they should be included. eg., the metrics should be included at api/v1/applications/<appId>/stages.

Metrics I've added are: submissionTime, firstTaskLaunchedTime and completionTime

Author: Xin Ren <iamshrek@126.com>

Closes #10107 from keypointt/SPARK-11155.