Add heartbeat command

[albireox]

Removed the automatic setting of the heartbeat variable. Added a heartbeat command that will be triggered by a heartbeat middleware.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 62.6%. Comparing base (81a4870) to head (160fa54).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
python/lvmecp/plc.py	20.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #28     +/-   ##
=======================================
+ Coverage   62.3%   62.6%   +0.4%     
=======================================
  Files         19      20      +1     
  Lines        881     883      +2     
  Branches     132     132             
=======================================
+ Hits         548     552      +4     
+ Misses       277     275      -2     
  Partials      56      56             

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