[PROPOSAL] Support OpenSearch Serverless

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What/Why

What are you proposing?

Amazon OpenSearch serverless was announced in 2022.

• [x] https://github.com/opensearch-project/opensearch-rs/pull/96
• [x] https://github.com/opensearch-project/opensearch-ruby/issues/131
• [x] https://github.com/opensearch-project/opensearch-java/issues/287
• [x] https://github.com/opensearch-project/opensearch-net/issues/129
• [x] https://github.com/opensearch-project/opensearch-js/issues/356
• [x] https://github.com/opensearch-project/opensearch-py/issues/259
• [x] https://github.com/opensearch-project/opensearch-go/issues/204
• [x] https://github.com/opensearch-project/opensearch-php/issues/113
• [x] https://github.com/acm19/aws-request-signing-apache-interceptor/issues/88
• [x] https://github.com/opensearch-project/logstash-output-opensearch/issues/186

What problems are you trying to solve?

Not all clients seem to work out of the box.

Are there any breaking changes to the API

OpenSearch Serverless has some differences. The first one is that / returns a 404 (info doesn't exist).

What is the user experience going to be?

• READMEs and COMPATIBILITY clearly state that the client works with OpenSearch Serverless
• USER_GUIDE has a section that explains the (minimal) differences

Why should it be built? Any reason not to?

Clients already support Sigv4, most should (almost) work.

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Closing this, decided not to include logstash-output-opensearch in the list of "clients".