Add backwards compatibility tests for automation

[VachaShah]

Add backwards compatibility tests for the plugin to automate end-to-end upgrade paths for faster releases. Developer documentation for implementing bwc tests and hooking them to CI: https://github.com/opensearch-project/opensearch-plugins/blob/main/TESTING.md#backwards-compatibility-testing

[praveensameneni]

closing the issue as the functionality is tested in plugins (e.g. index-management) using job-scheduler.

[dblock]

I'm reopening this. Job-scheduler is a plugin and it needs to have bcw tests all by itself.

[sbcd90]

closing the issue as pr is merged.