[INFRA] Serverless: Automated testing

[Swiddis]

Is your feature request related to a problem?
Coming from #125, it'd be nice to be able to test connectivity with OpenSearch Serverless.

What solution would you like?
N/A

What alternatives have you considered?
N/A

Do you have any additional context?
N/A

[dblock]

Serverless as Amazon OpenSearch Serverless?

We have a similar issue in opensearch-clients to test against vendor-specific (incl. Amazon) instances.
opensearch-project/opensearch-clients#47

[Catch All Triage - 1, 2, 3, 4, 5]