[PROPOSAL] Support OpenSearch Serverless

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What/Why

What are you proposing?

Amazon OpenSearch serverless was announced in 2022.

• Added support for Amazon OpenSearch Serverless. opensearch-rs#96
• [FEATURE] Add support for OpenSearch Serverless opensearch-ruby#131
• [FEATURE] Support OpenSearch Serverless request signing opensearch-java#287
• [FEATURE] Support for AWS OpenSearch Serverless Request Signing opensearch-net#129
• [FEATURE] Add support for OpenSearch Serverless opensearch-js#356
• [FEATURE] Add support for OpenSearch Serverless opensearch-py#259
• [FEATURE] Add support for OpenSearch Serverless opensearch-go#204
• [FEATURE] Add support for OpenSearch Serverless opensearch-php#113
• Support OpenSearch Serverless request signing acm19/aws-request-signing-apache-interceptor#88
• [BUG] - logstash-output-opensearch complains for cluster UUID while connecting to Amazon OpenSearch Serverless logstash-output-opensearch#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".