Add support for backup directive for VS and TS

[jjngx]

Proposed changes

This PR introduces support for backup directive (external name service) for Transport Server and Virtual Server for NIC Plus.

Checklist

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

• 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

[codecov]

Codecov Report

Attention: 61 lines in your changes are missing coverage. Please review.

Comparison is base (5e0bd5c) 51.96% compared to head (9942a71) 52.00%.

Files	Patch %	Lines
internal/k8s/controller.go	0.00%	30 Missing ⚠️
internal/configs/virtualserver.go	56.52%	15 Missing and 5 partials ⚠️
internal/configs/transportserver.go	68.00%	6 Missing and 2 partials ⚠️
pkg/apis/configuration/validation/virtualserver.go	91.89%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4653      +/-   ##
==========================================
+ Coverage   51.96%   52.00%   +0.03%     
==========================================
  Files          60       60              
  Lines       17191    17317     +126     
==========================================
+ Hits         8933     9005      +72     
- Misses       7954     7999      +45     
- Partials      304      313       +9     

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[ADubhlaoich]

LGTM still though I made a number of suggestions in the example README.

I think I got all instances, but for clarity:

• We only need to prepend F5 to the very first mention of NGINX Ingress Controller in a given piece of documentation.
• If we're referring to it as a product or a single instance, we just say "NGINX Ingress Controller": no need to prefix it with "the".

There's some other changes that could be made, but none are blocking, and I will make them myself in the future as part of reviewing the writing in the examples folder.