Add test coverage reporting

[dblock]

Add test coverage for unit and integration test reporting for all plugins.

[saratvemulapalli]

Linking the task from OpenSearch: opensearch-project/OpenSearch#850

[tlfeng]

Currently all the OpenSearch plugins that has got code coverage reporting are using JaCoCo to generate the code coverage report, and using Codecov for analysis and visualization.
OpenSearch Dashboards are using jest framework to generate code coverage for unit test.

[ylwu-amzn]

Is it enough to show test coverage percent on plugin readme doc?

[saratvemulapalli]

@ylwu-amzn yes that should be good for now, looks like most plugins have a badge which is used.

[saratvemulapalli]

Closing this as @tlfeng merged the changes to the repo.