Generate and upload code coverage report through CI workflow

[tlfeng]

All the plugins should have code coverage report uploaded to Codecov through their CI workflow.

So that the code coverage for each plugin repository can be seen in the Codecov dashboard for our organization: https://app.codecov.io/gh/opensearch-project/

Related issue: #41 #46

I will create issue in the specific plugin repositories as well.

[saratvemulapalli]

Thanks @tlfeng
Can you link the issues which are open here so its easier to track?

[tlfeng]

Thanks Sarat for your reminding. I created issues for the OpenSearch / Dashboards plugins that don't have got code coverage uploaded, and the plugins are got from the "Not yet setup" tab of Codecov dashboard (https://app.codecov.io/gh/opensearch-project/+).