Add jitter measurement

[amorphic]

Jitter is a measure of network connection consistency. This is important for activities such as streaming and gaming where an inconsistent connection can causing stuttering, buffering etc.

This card will be complete when a measurement exists which returns a network jitter result.

[amorphic]

As a first pass it may be easiest to add something like https://github.com/Griesbacher/check_jitter to the latency measurement.