Refactor watchers - Create separate type for namespaced informers

[ciarams87]

Proposed changes

Refactor namespaced watchers. This change creates a new namespacedInformer types in controller.go, and in the certmanager and externaldns controllers.

This simplifies the logic a bit, and allows breaking up larger methods into smaller methods/ functions.
Additionally, it also provides a base for creating new or deleting unused namespacedInformers during runtime, which could allow future dynamic updating of which namespaces are watched.

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 #3238 (10a5619) into main (42dd0d1) will increase coverage by 0.18%.
The diff coverage is 18.09%.

❗ Current head 10a5619 differs from pull request most recent head 7ec801c. Consider uploading reports for the commit 7ec801c to get more accurate results

@@            Coverage Diff             @@
##             main    #3238      +/-   ##
==========================================
+ Coverage   52.48%   52.67%   +0.18%     
==========================================
  Files          59       59              
  Lines       16104    16107       +3     
==========================================
+ Hits         8452     8484      +32     
+ Misses       7372     7341      -31     
- Partials      280      282       +2     

Impacted Files	Coverage Δ
internal/k8s/controller.go	11.27% <4.80%> (+0.32%)	⬆️
internal/externaldns/controller.go	6.20% <15.68%> (+6.20%)	⬆️
internal/certmanager/cm_controller.go	15.26% <35.89%> (-2.78%)	⬇️
internal/k8s/status.go	34.51% <43.58%> (+1.24%)	⬆️
internal/certmanager/helper.go	87.50% <53.33%> (-12.50%)	⬇️
internal/certmanager/sync.go	72.95% <100.00%> (-0.28%)	⬇️
internal/externaldns/sync.go	38.37% <100.00%> (+1.42%)	⬆️
... and 1 more

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[shaun-nx]

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