[SDK] Reset TraceFlags::IsSampled bit on sampler Decision::DROP

[marcalff]

Fixes #3725

Changes

Please provide a brief description of the changes here.

• When a sampler returns Decision::DROP or Decision::RECORD_ONLY, clear the trace sample flag, per specs.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.01%. Comparing base (6dcf647) to head (c7bac53).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3745      +/-   ##
==========================================
+ Coverage   90.01%   90.01%   +0.01%     
==========================================
  Files         225      225              
  Lines        7105     7106       +1     
==========================================
+ Hits         6395     6396       +1     
  Misses        710      710              

Files with missing lines	Coverage Δ
api/include/opentelemetry/trace/trace_flags.h	100.00% <ø> (ø)
sdk/src/trace/tracer.cc	83.88% <100.00%> (+0.27%)	⬆️

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