feat: enforce KongConsumer's credentials and consumerGroups uniqueness

[czeslavo]

What this PR does / why we need it:

I propose an alternative to the admission webhook-based approach that annotates credentials and consumerGroup with +listType=set which enforces the uniqueness of the elements in the list.

I verified the following scenario to ensure that migrating from the old CRD manifests to the new ones (with x-kubernetes-list-type: set) is allowed:

1. Create a fresh cluster, and apply old CRD manifests.
2. Create a couple of KongConsumers (both with duplicate and non-duplicate credentials/consumerGroups).
3. Apply the new CRD manifests (with x-kubernetes-list-type: set).
4. Ensure it works as expected:
   • Fetching the old KongConsumers works.
   • New KongConsumers violating the constraints are rejected.

Which issue this PR fixes:

Fixes #5402.

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]

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Additional details and impacted files

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[programmer04]

nice