chore: refactor code to decrease complexity

[odubajDT]

Fixes #533

Changes

• refactor code to remove nolint comments (where it makes sense)
• move common/constants package to common right next to the rest of constants
• increase test coverage for common package
• increase test coverage for FlagdProxyHandler
• polish flagdinjector

[codecov]

Codecov Report

Merging #554 (09ae2ee) into main (ccc0471) will increase coverage by 3.63%.
The diff coverage is 94.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
+ Coverage   84.27%   87.91%   +3.63%     
==========================================
  Files          10       10              
  Lines         935      927       -8     
==========================================
+ Hits          788      815      +27     
+ Misses        121       89      -32     
+ Partials       26       23       -3     

Files	Coverage Δ
apis/core/v1beta1/featureflagsource_types.go	96.59% <100.00%> (ø)
common/common.go	100.00% <ø> (ø)
common/flagdproxy/flagdproxy.go	86.98% <100.00%> (+2.05%)	⬆️
common/utils/utils.go	100.00% <ø> (+61.90%)	⬆️
webhooks/common.go	100.00% <100.00%> (ø)
webhooks/pod_webhook.go	85.85% <100.00%> (ø)
controllers/core/featureflagsource/controller.go	51.31% <83.33%> (+4.93%)	⬆️
common/flagdinjector/flagdinjector.go	92.06% <94.91%> (+1.83%)	⬆️

Flag	Coverage Δ
unit-tests	87.91% <94.59%> (+3.63%)	⬆️

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[beeme1mr]

There are a few variables that use improper casing of flagd. It would be nice to address them either in this PR or in a follow up.