简单实现中继负载均衡

[natsurainko]

PR-Codex overview

This PR focuses on enhancing the Relay system by introducing new settings, improving the relay management logic, and implementing a load reporting service for better performance tracking.

Detailed summary

• Added MaxReferenceConnectionCount and ServerPriority properties to IServerSettingProvider.
• Introduced RelayLoadManager for managing relay server availability.
• Implemented RelayLoadReportService for periodic load reporting.
• Updated GroupManager to assign relay servers based on availability.
• Refactored relay server assignment logic in GroupManager.
• Enhanced logging for relay manager actions and load reporting.
• Modified RelayManager to retrieve relay server load information.
• Improved session management and cleanup in RelayManager.

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