[FEAT] Add native support for Windows Service Control Handler

I am a heavy user of sing-box on Windows. Currently, to run sing-box as a background service, users have to rely on third-party wrappers like NSSM or WinSW. It would be a significant improvement if sing-box could support the native Windows Service Control Handler.

Why this is needed

1. Reduce External Dependencies: Currently, setting up sing-box as a service requires installing additional tools (like NSSM), which adds complexity and potential maintenance overhead for users.
2. Standardization: Native support would allow users to manage sing-box directly using standard Windows commands (e.g., sc start, sc stop) or the Services GUI (services.msc) without the need for an intermediary wrapper.
3. Better Reliability: Native integration ensures that the service lifecycle (start, pause, stop, and status reporting) is handled cleanly by the OS, leading to better stability.

Proposed Implementation

I understand that sing-box is designed to be cross-platform. Perhaps this could be implemented optionally or via a build tag for Windows, so that the core codebase remains clean and lightweight for other platforms.

Environment

OS: Windows 11
sing-box version: 1.14.0-alpha.31

Additional Context

Many users in the community currently use sc create or third-party wrappers, but they often encounter "service did not respond" errors because sing-box does not implement the ServiceMain and Handler callbacks required by the Windows SCM (Service Control Manager).

Thank you for your hard work on this incredible project! I look forward to hearing your thoughts on whether this fits into the future roadmap.

Begin by mapping the cross-platform startup path and the proposed Windows-only build boundary. The issue identifies the Windows SCM ServiceMain and Handler callbacks as the integration points; confirm how they should support start, pause, stop, and status reporting. Done means sing-box can be managed through sc commands or services.msc without NSSM or WinSW.