fix: validate configPatches for KongClusterPlugin

[randmonkey]

What this PR does / why we need it:

• Fix the validation of KongClusterPlugin to generate configuration using configPatches
• Do not share code of ValidateKongPlugin in ValidateKongClusterPlugin

Which issue this PR fixes:

fixes the problem of validator for KongClusterPlugin in #5158.

Special notes for your reviewer:

REVIEW: how do we write changelog here? This is a fix on a not yet released feature.

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: 4 lines in your changes are missing coverage. Please review.

Comparison is base (63253d7) 76.8% compared to head (e8880fe) 76.8%.
Report is 7 commits behind head on main.

Files	Patch %	Lines
internal/admission/validator.go	88.2%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5208     +/-   ##
=======================================
- Coverage   76.8%   76.8%   -0.1%     
=======================================
  Files        170     170             
  Lines      19050   19056      +6     
=======================================
- Hits       14649   14648      -1     
- Misses      3575    3578      +3     
- Partials     826     830      +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[czeslavo]

how do we write changelog here? This is a fix on a not yet released feature.

I'd say we just skip the changelog entry in such cases. 👍

[randmonkey]

how do we write changelog here? This is a fix on a not yet released feature.

I'd say we just skip the changelog entry in such cases. 👍

How about adding a link to this PR under the original entry?

[czeslavo]

How about adding a link to this PR under the original entry?

Yeah, it can work as well 👍

[randmonkey]

Found that we did not include tests on validation webhook of plugins (TestValidationWebhook only contains tests for validating credentials/consumers). Should we add them? If so, I'll create another issue.

[czeslavo]

Found that we did not include tests on validation webhook of plugins (TestValidationWebhook only contains tests for validating credentials/consumers). Should we add them? If so, I'll create another issue.

+1 for a separate issue.