feat(crd): enforce setting KongConsumer's either of username of custom_id via CRD validation rules

[pmalek]

What this PR does / why we need it:

According to https://docs.konghq.com/gateway/latest/admin-api/#consumer-object either of username or custom_id has to be set (both are also allowed) so update the CRD validation rules to enforce those rules.

Which issue this PR fixes:

Part of #5062

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

All modified and coverable lines are covered by tests ✅

Comparison is base (b4718ab) 77.8% compared to head (e593d65) 77.7%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5137     +/-   ##
=======================================
- Coverage   77.8%   77.7%   -0.1%     
=======================================
  Files        168     168             
  Lines      18907   18906      -1     
=======================================
- Hits       14714   14705      -9     
- Misses      3360    3366      +6     
- Partials     833     835      +2     

Files	Coverage Δ
internal/admission/validator.go	73.2% <ø> (+0.7%)	⬆️
internal/dataplane/kongstate/kongstate.go	86.7% <100.0%> (+<0.1%)	⬆️

... and 6 files with indirect coverage changes

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