fix: pass IPv6 DNS resolvers correctly

[sjberman]

Reopened from #4358 to workaround issues with forks at the moment.

Proposed changes

Problem: nginx expects IPv6 DNS resolvers to be passed with [] brackets:
invalid port in resolver \"2606:4700:4700::1111\" in /etc/nginx/stream-conf.d/stream.conf
But passing resolvers with brackets is not possible:
gatewayClassNp="&{Source:0xc0014382c0 ErrMsgs:[spec.dnsResolver.addresses[0].value: Invalid value: \"[2606:4700:4700::1111]\": must be a valid IP address] Valid:false}"

Solution: detect IPv6 resolvers and add brackets in the config templating.

Testing: deployed on an IPv6 capable cluster and configured DNS resolvers using the helm chart, deployed a gateway and http route pointing to an ExternalName service (which requires configuring a DNS resolver).

Closes #4369

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Fixed an issue regarding configuring IPv6 DNS resolvers for ExternalName services

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.14%. Comparing base (dbebd97) to head (d6c4148).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ternal/controller/nginx/config/base_http_config.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4378      +/-   ##
==========================================
+ Coverage   86.12%   86.14%   +0.01%     
==========================================
  Files         132      132              
  Lines       14343    14359      +16     
  Branches       35       35              
==========================================
+ Hits        12353    12369      +16     
  Misses       1786     1786              
  Partials      204      204              

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