Implement NNG JSON Publisher

[dbehnke]

This PR integrates a non-blocking NNG Publisher into the urfd core for real-time event broadcasting (connect, disconnect, and hearing events).

Features:

• Configurable NNG socket address via the [Dashboard] section in the .ini file.
• Thread-safe JSON serialization using nlohmann/json.
• Minimal overhead using NNG's non-blocking PUB pattern.

[dbehnke]

Implementation Overview: NNG JSON Publisher

This implementation adds a real-time event broadcasting system to the urfd core using the NNG (Nanomsg Next Gen) library.

Why NNG?

Current telemetry in urfd (JSON/XML files) is periodic and "pull-based," which introduces latency for live dashboards. NNG’s PUB/SUB pattern allows the reflector to "push" events instantly to any number of subscribers (dashboard middleware, logging services, etc.) with minimal overhead.

Technical Highlights

• Asynchronous & Non-blocking: Broadcasts use the NNG_FLAG_NONBLOCK flag. This ensures that the core reflector logic remains completely decoupled from the telemetry consumers; if a consumer is slow or disconnected, the reflector is never blocked.
• Centralized Event Hooks: By hooking into the CClients and CUsers containers, we capture connection and "Hearing" (Last Heard) events across all supported protocols (M17, YSF, DMR, D-Star, etc.) simultaneously.
• Configurable Binding: The publisher address is configurable in the .ini file, allowing it to bind to TCP or IPC (Unix Domain Sockets) based on the deployment environment.
• Standardized JSON: Leverages the existing nlohmann/json dependency to provide a consistent, easily consumable data format for modern web-based dashboards.

This addition provides the foundation for low-latency, real-time monitoring of reflector activity.