Add passthrough for gRPC connection when using a proxy

[dhurley]

Proposed changes

Add passthrough for gRPC connection when using a proxy. This is for scenarios where NGINX Agent has no internet access and can't resolve the hostname of the management server itself.

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 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.41%. Comparing base (012b734) to head (7abc12b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/grpc/grpc.go	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1375      +/-   ##
==========================================
+ Coverage   86.28%   86.41%   +0.12%     
==========================================
  Files         102      102              
  Lines       12572    12580       +8     
==========================================
+ Hits        10848    10871      +23     
+ Misses       1248     1231      -17     
- Partials      476      478       +2     

Files with missing lines	Coverage Δ
internal/grpc/grpc.go	83.25% <92.30%> (+0.61%)	⬆️

... and 5 files with indirect coverage changes

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