[repo] Add THIRD-PARTY-NOTICES file

[CodeBlanch]

Relates to #5187

Changes

• Adds THIRD-PARTY-NOTICES file to include the dotnet\runtime MIT license info for the files we internalized (in src\Shared\EnvironmentVariables & src\Shared\Options)

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (65dd83b) 83.47% compared to head (a539a59) 83.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5190      +/-   ##
==========================================
+ Coverage   83.47%   83.49%   +0.01%     
==========================================
  Files         297      297              
  Lines       12397    12398       +1     
==========================================
+ Hits        10349    10352       +3     
+ Misses       2048     2046       -2     

Flag	Coverage Δ
unittests	83.49% <ø> (+0.01%)	⬆️

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see 6 files with indirect coverage changes