Add OpenSearch Serverless (AOSS) support

[norrishuang]

Summary

Add support for Amazon OpenSearch Serverless (AOSS) to the awsopensearch client.

Changes

• config.py: Add is_serverless and aws_region fields; use AWS SigV4 authentication via requests-aws4auth for AOSS connections
• cli.py: Add --serverless flag and --aws-region option
• aws_opensearch.py: Adapt client for AOSS constraints:
  • Skip unsupported operations (cluster settings, force merge, manual refresh, replica updates, warmup API)
  • Use smaller batch size (100) for bulk inserts
  • Store id as a document field (AOSS doesn't support custom _id)
  • Retrieve id from _source in search results
  • Remove engine and encoder from method config (AOSS manages internally)
  • Disable http_compress to avoid SigV4 checksum verification failures
• README.md: Add OpenSearch Serverless section with usage example and notes

Usage

vectordbbench awsopensearch --db-label aoss \
  --serverless --aws-region us-east-1 \
  --host <collection-id>.aoss.us-east-1.on.aws --port 443 \
  --case-type Performance768D1M \
  --m 16 --ef-construction 200 --ef-search 40 \
  --number-of-shards 8 --number-of-replicas 0 \
  --engine faiss --metric-type cosine \
  --num-concurrency 80,100,120

Prerequisites

• AWS credentials configured
• requests-aws4auth installed
• IAM identity policy with aoss:APIAccessAll
• AOSS Data Access Policy granting index/collection permissions

Testing

Tested against a live AOSS collection with 1M 768-dim vectors (Cohere dataset). Data loading and index creation verified successfully.

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