Add support for OTEL_SDK_DISABLED environment variable

[hannahhaering]

Implements #4155

Changes

• Implemented OTEL_SDK_DISABLED according to the OpenTelemetry specification
• Applied support across TracerProvider, MeterProvider, and LoggerProvider

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.61%. Comparing base (6a70665) to head (583b34b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6568      +/-   ##
==========================================
- Coverage   86.70%   86.61%   -0.09%     
==========================================
  Files         258      258              
  Lines       11895    11910      +15     
==========================================
+ Hits        10313    10316       +3     
- Misses       1582     1594      +12     

Flag	Coverage Δ
unittests-Project-Experimental	86.31% <100.00%> (-0.07%)	⬇️
unittests-Project-Stable	86.54% <100.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...elemetry/Logs/Builder/LoggerProviderBuilderBase.cs	97.36% <100.00%> (+0.39%)	⬆️
...emetry/Metrics/Builder/MeterProviderBuilderBase.cs	97.56% <100.00%> (+0.33%)	⬆️
...lemetry/Trace/Builder/TracerProviderBuilderBase.cs	98.24% <100.00%> (+0.16%)	⬆️

... and 5 files with indirect coverage changes