NGINX Configs to Reference Remote External Files

[Akshay2191]

Extends the ConfigApply capability to manage NGINX configurations that reference external resources hosted at remote URLs. The agent now handles downloading of these files into the NGINX configuration directory before applying the new configuration.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[codecov]

Codecov Report

❌ Patch coverage is 31.36364% with 151 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.49%. Comparing base (b1947ec) to head (501e08f).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/file/file_manager_service.go	15.43%	134 Missing and 3 partials ⚠️
internal/file/file_service_operator.go	57.14%	9 Missing ⚠️
internal/config/config.go	86.48%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1389      +/-   ##
==========================================
- Coverage   86.31%   85.49%   -0.83%     
==========================================
  Files         102      102              
  Lines       12603    12792     +189     
==========================================
+ Hits        10878    10936      +58     
- Misses       1249     1376     +127     
- Partials      476      480       +4     

Files with missing lines	Coverage Δ
internal/config/defaults.go	100.00% <ø> (ø)
internal/config/flags.go	100.00% <ø> (ø)
internal/config/types.go	87.65% <ø> (ø)
internal/model/config.go	90.47% <ø> (ø)
internal/config/config.go	87.27% <86.48%> (-0.03%)	⬇️
internal/file/file_service_operator.go	74.84% <57.14%> (-1.30%)	⬇️
internal/file/file_manager_service.go	56.32% <15.43%> (-19.07%)	⬇️

... and 5 files with indirect coverage changes

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