Integrate with checks-api plugin

[timja]

This will allow publishing test results to GitHub checks

See https://github.com/jenkinsci/checks-api-plugin/blob/master/docs/consumers-guide.md

and jenkinsci/warnings-ng-plugin#550

This should be updated in the JunitStep https://github.com/jenkinsci/junit-plugin/blob/master/src/main/java/hudson/tasks/junit/pipeline/JUnitResultsStep.java

[timja]

cc @XiongKezhi in case you want to add anything

[XiongKezhi]

LGTM now. I can help with it after this hiring season if still nobody is interested in it by then.

[KalleOlaviNiemitalo]

FYI, I filed JENKINS-64714 requesting a similar feature in the xUnit plugin.