fetch Kong entity schema and validate KongCustomEntity resources

[randmonkey]

What this PR does / why we need it:

Fetch entity schema and validate KongCustomEntity against Kong gateway.
Added unit tests and envtest tests to verify the parsing of entity schema and validations.

Which issue this PR fixes:

fixes #5975

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: Patch coverage is 58.33333% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 52.3%. Comparing base (d8c30ad) to head (aeadb93).
Report is 3 commits behind head on main.

❗ Current head aeadb93 differs from pull request most recent head 0886d34. Consider uploading reports for the commit 0886d34 to get more accurate results

Files	Patch %	Lines
internal/admission/handler.go	0.0%	11 Missing ⚠️
internal/admission/validator.go	70.3%	5 Missing and 3 partials ⚠️
internal/dataplane/kongstate/customentity.go	79.3%	3 Missing and 3 partials ⚠️
internal/admission/adminapi_provider.go	0.0%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #6006      +/-   ##
========================================
- Coverage   74.0%   52.3%   -21.7%     
========================================
  Files        182     183       +1     
  Lines      18345   18417      +72     
========================================
- Hits       13582    9644    -3938     
- Misses      3825    8138    +4313     
+ Partials     938     635     -303     

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