Comprehensive string constant optimization across curvine-s3-gateway:

• Create utils/consts.rs with 60+ string constants
• Optimize S3 API constants (storage classes, content types, metadata keys)
• Optimize HTTP constants (query parameters, headers, paths)
• Optimize auth constants (AWS signatures, headers, error messages)
• Optimize error handling constants (error display messages)
• Replace 100+ hardcoded strings with named constants
• Improve code maintainability and reduce magic strings
• All optimizations compile without warnings
• Maintain 100% functional compatibility

Files modified:

• utils/: consts.rs (new), mod.rs, s3_utils.rs
• auth/: sig_v4.rs
• http/: router.rs, axum.rs
• s3/: s3_api.rs, error.rs, types.rs
• lib.rs (root module)