Allow the heartbeat timer to be disabled by setting it to None

[davidsteiner]

Rather than pushing out the duration to a large amount, make the heartbeat timer an Option so we can just disable it by setting it to None

[codecov]

Codecov Report

❌ Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.31%. Comparing base (11d327c) to head (da41f56).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/src/session.rs	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   60.04%   62.31%   +2.26%     
==========================================
  Files          66       66              
  Lines        2723     2749      +26     
==========================================
+ Hits         1635     1713      +78     
+ Misses       1088     1036      -52     

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