feat: Add full S3 backend agent as a standalone package in pkg/s3

[Venksaiabhishek]

• Includes full Go S3/Contexture agent code in pkg/s3/ (AWS adapters, Contexture OCS engine, tools, api)
• Add S3 onboarding CLI tool (cmd/s3-onboarding/) for verifying MinIO connectivity
• Add S3 configuration file (config/s3_config.yaml)
• Fix unreachable dead code in client.py
• Fix duplicate httpx import in client.py
• Fix hardcoded model name in client.py (use MODEL_NAME constant)
• Fix typo in server.py: promotheus -> prometheus
• Update .gitignore to exclude Go binaries, tar.gz, and log files

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