[CONFIGURATION] Support environment variable substitution for attributes

[dbarker]

Fixes #4473

Changes

• Add null value detection to the ryml document
• Add env var substitution to all attribute value type parsing methods
• Add unit tests and a programmatic test for the use case in the linked issue

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 85.71429% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.06%. Comparing base (1c2b007) to head (828de82).

Files with missing lines	Patch %	Lines
sdk/src/configuration/configuration_parser.cc	73.34%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4474      +/-   ##
==========================================
+ Coverage   82.67%   83.06%   +0.40%     
==========================================
  Files         516      516              
  Lines       20197    20224      +27     
==========================================
+ Hits        16696    16798     +102     
+ Misses       3501     3426      -75     

Files with missing lines	Coverage Δ
...de/opentelemetry/sdk/configuration/document_node.h	100.00% <ø> (ø)
...entelemetry/sdk/configuration/ryml_document_node.h	100.00% <ø> (ø)
sdk/src/configuration/ryml_document_node.cc	83.59% <100.00%> (+3.19%)	⬆️
sdk/src/configuration/sdk_builder.cc	61.21% <100.00%> (+0.04%)	⬆️
sdk/src/configuration/configuration_parser.cc	84.86% <73.34%> (+4.97%)	⬆️

... and 1 file with indirect coverage changes

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[marcalff]

LGTM, good work.