yaml config env var spec conformance

[brettmc]

Updating env var substitution to conform to https://github.com/open-telemetry/opentelemetry-specification/blob/v1.41.0/specification/configuration/data-model.md#environment-variable-substitution

• env: prefix allowed in environment variable
• do not recursively apply env var substitution (ie don't allow env vars in env vars)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.83%. Comparing base (7eb3abb) to head (162f22e).

Files with missing lines	Patch %	Lines
...guration/Internal/EnvSubstitutionNormalization.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1486      +/-   ##
============================================
+ Coverage     70.71%   70.83%   +0.11%     
  Complexity     2761     2761              
============================================
  Files           408      408              
  Lines          8341     8341              
============================================
+ Hits           5898     5908      +10     
+ Misses         2443     2433      -10     

Flag	Coverage Δ
8.1	70.45% <0.00%> (+0.01%)	⬆️
8.2	70.66% <0.00%> (+0.09%)	⬆️
8.3	70.66% <0.00%> (+0.01%)	⬆️
8.4	70.62% <0.00%> (+0.04%)	⬆️
8.5	70.69% <0.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
...guration/Internal/EnvSubstitutionNormalization.php	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

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