[CONFIGURATION] File configuration - extension model

[marcalff]

Contributes to #2481

This is a partial fix, that implements supporting declarations.

Changes

Please provide a brief description of the changes here.

• Implement the extension model for file configuration.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	6bc4bc9
🔍 Latest deploy log	https://app.netlify.com/projects/opentelemetry-cpp-api-docs/deploys/686843b1b9ea1700085b92d9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.95%. Comparing base (a91d6a1) to head (6bc4bc9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3503      +/-   ##
==========================================
- Coverage   89.96%   89.95%   -0.01%     
==========================================
  Files         219      219              
  Lines        7051     7051              
==========================================
- Hits         6343     6342       -1     
- Misses        708      709       +1     

see 1 file with indirect coverage changes

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[dbarker]

LGTM