feat: add ping scheduler for node liveness checks

[roganlynch]

Adds a ping scheduler for automated node liveness checks.

Changes

• PingScheduler: Manages periodic ping operations for node health monitoring
• Configurable intervals: Supports customizable ping schedules per node
• Automatic cleanup: Handles timeout and response cleanup
• Integration with PingManager: Uses the core ping infrastructure for execution

Implementation Details

The scheduler maintains a registry of nodes requiring periodic health checks and coordinates ping request timing. It integrates with the PingManager to execute pings and process responses, providing automated liveness monitoring for the network.

Context

Part of the work discussed in #2902 to add ping functionality to headscale.

Dependencies

• Requires PR #2939 (core ping implementation)

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Note: This code was generated with assistance from Claude Sonnet 4.5 via cline.bot