support OpenSearch 1.x in non-compatibility mode and 2.x

Coming from https://github.com/opensearch-project/OpenSearch/issues/7280 it looks like neptune tools may need to add support for OpenSearch. The existing Elasticsearch tools should work well for OpenSearch 1.x in backwards compatibility mode (haven't tested), but 2.x has [breaking changes](https://opensearch.org/docs/2.0/breaking-changes/). 