Add method to be able to send heartbeat

[wnagele]

Allows to check if the connection is still alive and working. I need this in a case where other calls are only happening fairly infrequently and a broken connection would only show up much later.

[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.

Project coverage is 61.64%. Comparing base (abf9e96) to head (d77335c).
Report is 3 commits behind head on master.

Files	Patch %	Lines
meshtastic/mesh_interface.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
+ Coverage   61.63%   61.64%   +0.01%     
==========================================
  Files          24       24              
  Lines        3597     3598       +1     
==========================================
+ Hits         2217     2218       +1     
  Misses       1380     1380              

Flag	Coverage Δ
unittests	61.64% <11.11%> (+0.01%)	⬆️

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[ianmcorvidae]

This looks good to me (we can ignore the code coverage aspect, I think)! I know a few others have been interested in something like this being split out for easy use.