This uses a docker image (built from https://gitlab.wikimedia.org/repos/search-platform/cirrussearch-elasticsearch-image) to run elasticsearch. Put together a comparable image for opensearch and get it working with the integration testing environment. Initially this can leave custom plugins disabled, Cirrus is designed to work without them.
Probably needs a new branch created in CirrusSearch to hold necessary changes. Probably requires forking Elastica somewhere (perhaps into the Elastica extension repo?) and making other necessary changes to allow communication.