adding OTEL_SDK_DISABLED support

[brettmc]

Implementing the recently-added OTEL_SDK_DISABLED variable: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#general-sdk-configuration

[codecov]

Codecov Report

Merging #833 (0cde756) into main (215731f) will decrease coverage by 0.05%.
The diff coverage is 60.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #833      +/-   ##
============================================
- Coverage     82.54%   82.49%   -0.06%     
- Complexity     1877     1883       +6     
============================================
  Files           232      234       +2     
  Lines          4893     4901       +8     
============================================
+ Hits           4039     4043       +4     
- Misses          854      858       +4     

Flag	Coverage Δ
7.4	82.50% <60.00%> (-0.06%)	⬇️
8.0	82.52% <60.00%> (-0.06%)	⬇️
8.1	82.52% <60.00%> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
src/SDK/Trace/NoopTracerProvider.php	0.00% <0.00%> (ø)
src/SDK/Metrics/MeterProvider.php	90.90% <100.00%> (ø)
src/SDK/SDK.php	100.00% <100.00%> (ø)
src/SDK/Trace/TracerProvider.php	100.00% <100.00%> (ø)
src/SDK/Trace/TracerProviderFactory.php	100.00% <100.00%> (ø)

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