feat: Support watching multiple namespaces

[ciarams87]

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Proposed changes

Currently, the NGINX Ingress Controller can watch either all namespaces or a single namespace. This commit adds the ability to watch multiple namespaces (defined using by passing a comma separated list of namespaces to the -watch-namespace CLI argument instead of a single name). This is achieved by creating an informer per namespace and kind we observe.

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-commenter]

Codecov Report

Merging #2914 (ebe60f4) into main (094b877) will decrease coverage by 0.72%. The diff coverage is 14.61%.

@@            Coverage Diff             @@
##             main    #2914      +/-   ##
==========================================
- Coverage   53.03%   52.30%   -0.73%     
==========================================
  Files          58       58              
  Lines       15651    15959     +308     
==========================================
+ Hits         8300     8347      +47     
- Misses       7074     7332     +258     
- Partials      277      280       +3     

Impacted Files	Coverage Δ
cmd/nginx-ingress/main.go	0.00% <0.00%> (ø)
internal/externaldns/controller.go	0.00% <0.00%> (ø)
internal/k8s/controller.go	11.18% <4.65%> (-0.53%)	:arrow_down:
internal/k8s/status.go	33.27% <23.63%> (-0.97%)	:arrow_down:
internal/certmanager/cm_controller.go	18.04% <40.00%> (-0.76%)	:arrow_down:
cmd/nginx-ingress/flags.go	34.42% <75.00%> (+8.50%)	:arrow_up:
internal/externaldns/sync.go	36.95% <75.00%> (+0.64%)	:arrow_up:
internal/certmanager/sync.go	73.23% <100.00%> (+1.12%)	:arrow_up:
...ternal/k8s/appprotect/app_protect_configuration.go	86.16% <0.00%> (-0.58%)	:arrow_down:
... and 5 more

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