[Build] Add bwc testing w/ 1.x and 2.x fixture

[nknize]

Is your feature request related to a problem?

opensearch-hadoop is written to support 2.x, and recently 1.x, but there is no bwc test fixture support. This is a problem if we want to verify backwards compatibility and prevent breaking changes.

What solution would you like?

Add support for bwcTest similar to the core repo. For now we should be able to write a bwc-test.gradle file similar to the core repo and leverage InternalBwcGitPlugin and InternalDistributionDownloadPlugin from build-tools dependency, but longer term core might be separating the build infrastructure into internal and extensible build tools so we'd want this setup for an easy transition if/when that happens.

What alternatives have you considered?

Do nothing. I don't think this is an alternative if we want to support bwc. (which we do)

Do you have any additional context?

The community really wants this connector so we need to ensure stability and quality.