Changes

Add a new postsubmit task for tikv to report the code coverage of the tikv repository (#4003)

Change the original jenkins task cron_tikv_ghpr_coverage on
ci.pingcap.net to use the native Prow job format and port it to a
post-submit task.

Original jenkins task:
- https://ci.pingcap.net/job/cron_tikv_ghpr_coverage/

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: wuhuizuo <wuhuizuo@126.com>
 (commit: eed2a37)