Add support for AWS OpenSearch Serverless domains

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Earlier this year, AWS OpenSearch Serverless went GA. For self-managed Gitlab instances with a relatively small number projects, Serverless looks like a more cost-effective solution than a managed AWS OpenSearch domain. The AWS OpenSearch documentation has a breakdown of the differences between managed domains and serverless domains. It looks like the main blocker to using the indexer with Serverless is the change to request signing.