feat: expose RTT jitter in Statistics struct

[davidebettio]

This PR adds support for tracking RTT jitter in the Pinger struct and exposes it via the Statistics struct.

What’s included:

• Added jitter calculation based on mean absolute differences between consecutive RTTs.
• New field Jitter time.Duration in both Pinger and Statistics.
• Automatically updated during updateStatistics if RecordRtts is enabled.
• Some unit tests to verifying the correctness of jitter computation.