feat(consumer group): add validation admission webhook

[programmer04]

What this PR does / why we need it:

Adds validation webhook for consumer groups that work this way

• With an Enterprise Gateway with a license -> OK
• With a Gateway that is too old to support consumer groups -> Error + block
• With a Free mode Gateway -> Error (info about invalid/missing license) + block
• With an OSS gateway -> Error + block

Which issue this PR fixes:

This is part of #3728

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

Patch coverage: 57.3% and project coverage change: -1.8% ⚠️

Comparison is base (e686d34) 67.6% compared to head (22e7789) 65.8%.

❗ Current head 22e7789 differs from pull request most recent head b1e9b3d. Consider uploading reports for the commit b1e9b3d to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4437     +/-   ##
=======================================
- Coverage   67.6%   65.8%   -1.8%     
=======================================
  Files        160     155      -5     
  Lines      18742   18670     -72     
=======================================
- Hits       12670   12298    -372     
- Misses      5309    5654    +345     
+ Partials     763     718     -45     

Files Changed	Coverage Δ
internal/admission/handler.go	35.2% <0.0%> (-2.7%)	⬇️
internal/admission/validator.go	36.4% <64.8%> (+0.2%)	⬆️
internal/admission/adminapi_provider.go	96.8% <91.6%> (-3.2%)	⬇️

... and 35 files with indirect coverage changes

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