sdk autoloading

[brettmc]

Enable autoloading/creating a global SDK from environment variables, which can be used by auto-instrumentation.
This should allow auto-instrumentation without any code (with env vars). Note the addition of a new env var, OTEL_PHP_AUTOLOAD_ENABLED - it must be enabled to allow autoloading.

[codecov]

Codecov Report

Merging #854 (db1e438) into main (f083b10) will decrease coverage by 0.58%.
The diff coverage is 54.16%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #854      +/-   ##
============================================
- Coverage     81.25%   80.67%   -0.59%     
- Complexity     1971     2011      +40     
============================================
  Files           250      255       +5     
  Lines          5165     5279     +114     
============================================
+ Hits           4197     4259      +62     
- Misses          968     1020      +52     

Flag	Coverage Δ
7.4	80.17% <54.16%> (-0.58%)	⬇️
8.0	80.67% <54.16%> (-0.59%)	⬇️
8.1	80.81% <54.16%> (-0.59%)	⬇️

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Impacted Files	Coverage Δ
src/Contrib/Otlp/MetricExporterFactory.php	0.00% <0.00%> (ø)
src/Contrib/Otlp/Protocols.php	0.00% <ø> (ø)
src/SDK/Metrics/MeterProviderBuilder.php	0.00% <0.00%> (ø)
.../SDK/Metrics/MetricExporter/NoopMetricExporter.php	0.00% <0.00%> (ø)
src/SDK/SdkBuilder.php	82.14% <0.00%> (ø)
src/SDK/SdkAutoloader.php	43.47% <43.47%> (ø)
src/SDK/Metrics/MeterProviderFactory.php	81.48% <81.48%> (ø)
src/SDK/Propagation/PropagatorFactory.php	85.71% <85.71%> (ø)
src/Contrib/Zipkin/SpanConverter.php	96.29% <100.00%> (ø)
src/SDK/Common/Configuration/Configuration.php	100.00% <100.00%> (ø)
... and 3 more

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[brettmc]

This should wait for #859; it will need to incorporate config changes from that PR