Add dynamic configmap informer factory

[tcnghia]

Changes

• 🎁 Add configmap informer factory

#2210 introduced a new manually crafted namespacedkube implementation for secrets. This PR adds a similar factory for configmap.

Release Note
This change has no user-visible impact.

Docs
This change has no user-visible impact.

[codecov]

Codecov Report

Merging #2466 (e5ec596) into main (13bdd74) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2466      +/-   ##
==========================================
- Coverage   63.55%   63.53%   -0.02%     
==========================================
  Files         228      228              
  Lines       10007    10007              
==========================================
- Hits         6360     6358       -2     
- Misses       3344     3345       +1     
- Partials      303      304       +1     

Impacted Files	Coverage Δ
test/gcs/mock/mock.go	90.32% <0.00%> (-1.08%)	⬇️

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[mattmoor]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattmoor, tcnghia

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