[SDK] Support TracerConfigurator updates

[dbarker]

Add support for updating the TracerConfigurator at runtime and provide an example.

The TracerProvider configuration includes the TracerConfigurator. SDKs may support updating the TracerConfigurator and resulting TracerConfig objects at runtime provided it is applied to previously created tracers. This feature allows dynamic enabling of tracers to turn tracing on or off without restarting the application.

https://opentelemetry.io/docs/specs/otel/trace/sdk/#configuration

The TracerProvider MAY provide methods to update the configuration. If configuration is updated (e.g., adding a SpanProcessor), the updated configuration MUST also apply to all already returned Tracers (i.e. it MUST NOT matter whether a Tracer was obtained from the TracerProvider before or after the configuration change).

Changes

• Add SDK TracerProvider::UpdateTracerConfigurator method and tests
• Change SDK Tracer::tracer_config_ to be an atomic shared pointer that will support dynamic updates including growth beyond the enabled bool.
• Add example showing runtime changes to the tracer configurator to enable/disable tracers.

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

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.00%. Comparing base (e0c6c33) to head (7bf4ca3).

Files with missing lines	Patch %	Lines
sdk/src/trace/tracer_context.cc	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4065      +/-   ##
==========================================
+ Coverage   81.99%   82.00%   +0.01%     
==========================================
  Files         385      386       +1     
  Lines       16023    16043      +20     
==========================================
+ Hits        13137    13155      +18     
- Misses       2886     2888       +2     

Files with missing lines	Coverage Δ
sdk/include/opentelemetry/sdk/trace/tracer.h	100.00% <ø> (ø)
...k/include/opentelemetry/sdk/trace/tracer_context.h	100.00% <ø> (ø)
sdk/src/trace/tracer.cc	87.02% <100.00%> (+0.91%)	⬆️
sdk/src/trace/tracer_provider.cc	90.91% <100.00%> (+1.33%)	⬆️
sdk/src/trace/tracer_context.cc	80.65% <60.00%> (-3.97%)	⬇️

... and 1 file with indirect coverage changes

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