feat: add validation on secrets on generated plugin cofigs

[randmonkey]

What this PR does / why we need it:

validates secrets when they are referred by KongPlugin and KongClusterPlugin to check if they generate invalid Kong plugin configurations.

Which issue this PR fixes:

fixes #5190

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (a4f82f6) 75.9% compared to head (f651698) 75.8%.

Files	Patch %	Lines
internal/admission/handler.go	67.9%	13 Missing and 4 partials ⚠️
internal/controllers/reference/indexer.go	55.5%	8 Missing and 4 partials ⚠️
internal/manager/run.go	66.6%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5203     +/-   ##
=======================================
- Coverage   75.9%   75.8%   -0.1%     
=======================================
  Files        170     170             
  Lines      19447   19549    +102     
=======================================
+ Hits       14767   14837     +70     
- Misses      3851    3872     +21     
- Partials     829     840     +11     

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