[REFACTOR] Move BWC tests to their own gradle build

[dbwiddis]

Coming from #551 (comment)

Is your feature request related to a problem?

The build.gradle file is getting big and confusing to navigate.

What solution would you like?

Change our BWC setup (derived from AD and ML Commons setup) to the Security repo pattern.

See https://github.com/opensearch-project/security/blob/main/bwc-test/build.gradle

What alternatives have you considered?

Leave it as is, to reduce a lot of the duplication that will be required.

Refactor to a completely different model as some other plugins are doing.

Do you have any additional context?

It took several hours to get this working. I don't expect this will be a trivial exercise to move it.

Other possibilities mentioned in #551 review:

• try to run on Windows
• simplify some code in BWC test (and submit similar changes to AD and ML Commons)

[amitgalitz]

KNN also has a good example of separating rolling and restart upgrade somewhere else: https://github.com/opensearch-project/k-NN/blob/2.x/qa/rolling-upgrade/build.gradle