make SdkTracer.tracerEnabled mutable

[jackshirazi]

This allows the tracer to become enabled and disabled dynamically (though only by reflection at this stage). Doing just this change allows downstream agent distros to play around with APIs that would best work while not yet changing anything public in the SDK
Without this change, testing downstream is still possible but much more effortful

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.02%. Comparing base (5ca6177) to head (1421bb6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6687      +/-   ##
============================================
+ Coverage     90.00%   90.02%   +0.01%     
- Complexity     6332     6353      +21     
============================================
  Files           703      703              
  Lines         19092    19138      +46     
  Branches       1883     1887       +4     
============================================
+ Hits          17184    17229      +45     
- Misses         1332     1333       +1     
  Partials        576      576              

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