Add feature for threshold ecdh psi

[yunbo99]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @yunbo99, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces the Threshold ECDH Private Set Intersection (PSI) feature, which enhances the standard ECDH PSI protocol by allowing participants to specify a maximum size for the resulting intersection. This ensures that the final intersection count does not exceed a predefined threshold, providing an additional layer of control over data exposure. The implementation includes new C++ components for both sender and receiver roles, along with updated configuration structures and examples, making it a robust extension to the existing PSI framework.

Highlights

• Threshold ECDH PSI Implementation: Introduced a new feature for Threshold ECDH Private Set Intersection (PSI), which allows users to specify a maximum size for the intersection result, enhancing privacy controls.
• Configuration Extensions: Added new configuration fields, intersection_threshold and count_path, to the PsiConfig protobuf message to support the new threshold functionality.
• New C++ Components: Developed dedicated C++ classes including ThresholdEcdhPsiReceiver, ThresholdEcdhPsiSender, ThresholdEcdhPsiCtx, and ThresholdEcdhPsiCache to handle the specific logic and data management for the threshold-based protocol.
• Dynamic Protocol Selection: Integrated the new Threshold ECDH PSI into the psi_launcher application, enabling dynamic selection between standard ECDH PSI and the threshold-aware version based on the provided configuration.
• Base Class Refactoring: Modified the access specifiers of key methods in the base EcdhPsiReceiver and EcdhPsiSender classes from private to protected to facilitate proper inheritance and extension by the new threshold-specific implementations.

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[gemini-code-assist]

Code Review

This pull request introduces a significant new feature: Threshold ECDH PSI, which allows limiting the size of the intersection result. The implementation is well-structured, extending the existing ECDH PSI functionality through inheritance. The changes include new protocol logic, configuration options, comprehensive tests, benchmarks, and documentation. The overall approach is solid. I've identified a few areas for improvement, such as correcting some copy-paste errors in logging, removing unused code, improving code clarity, and fixing typos in comments. These suggestions should help enhance the quality and maintainability of the new feature.