feat: Enable configuration of ignored and expected HTTP status code errors with environment variables

[nr-ahemsath]

Description

This PR resolves #2181 by introducing two new environment variables for configuring agent behavior:
NEW_RELIC_ERROR_COLLECTOR_IGNORE_ERROR_CODES
NEW_RELIC_ERROR_COLLECTOR_EXPECTED_ERROR_CODES

Both env vars support defining multiple HTTP status codes in a comma-separated list, e.g.:
NEW_RELIC_ERROR_COLLECTOR_IGNORE_ERROR_CODES="401,503"

The env var for expected error codes also supports defining a range of codes like this:
NEW_RELIC_ERROR_COLLECTOR_EXPECTED_ERROR_CODES="401,500-505"

The internal logic of the error collector doesn't currently support ranges for ignored error codes, hence the difference.

I will update the description of this PR with a link to the docs site update PR once that is created.

Author Checklist

• Unit tests, Integration tests, and Unbounded tests completed
• Performance testing completed with satisfactory results (if required) N/A

Reviewer Checklist

• Perform code review
• Pull request was adequately tested (new/existing tests, performance tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.60%. Comparing base (c358d21) to head (68125ad).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2487      +/-   ##
==========================================
+ Coverage   80.57%   80.60%   +0.02%     
==========================================
  Files         459      459              
  Lines       28777    28774       -3     
  Branches     3184     3184              
==========================================
+ Hits        23186    23192       +6     
+ Misses       4805     4798       -7     
+ Partials      786      784       -2     

Flag	Coverage Δ
Agent	81.54% <100.00%> (+0.03%)	⬆️
Profiler	72.15% <ø> (+<0.01%)	⬆️

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Files	Coverage Δ
...c/Agent/Core/Configuration/DefaultConfiguration.cs	89.17% <100.00%> (+0.31%)	⬆️

... and 3 files with indirect coverage changes