Add backwards compatibility tests for automation

[zelinh]

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

This would include the BWC tests on both plugin level and distribution bundle level.

[praveensameneni]

plan to target for 2.15

[zelinh]

Can we get some traction on this issue? This BWC tests request would be a future release blocker.

[zelinh]

@eirsep @sbcd90 Do we have a plan to onboard to this distribution level BWC test? We are planning to add BWC tests as mandatory exit criteria in future releases.