feat: Added heartbeat at each frame prediction

[frgfm]

This PR simply adds a heartbeat (ping) at each frame prediction when the frame sender is known.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #59 (764fe00) into master (f271ad6) will decrease coverage by 0.44%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   63.90%   63.46%   -0.45%     
==========================================
  Files          10       10              
  Lines         205      208       +3     
==========================================
+ Hits          131      132       +1     
- Misses         74       76       +2     

Flag	Coverage Δ
unittests	63.46% <33.33%> (-0.45%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyroengine/engine/engine.py	53.42% <33.33%> (-0.87%)	⬇️

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[Akilditu]

Thanks for the PR !

This will allow us to report on heartbeats logs and build a dashboard to monitor devices activity.

Quick note : if there is no more heartbeat on one or few pizeros linked to the same tower it means that the issue may be pizero-related. If there is no signal for all pizero on a same tower it means that the issue is more likely related to the main rpi / 4G hotspot.