feat(kafka_schema): added kafka schema references

[vmyroslav]

About this change—what it does

• Added kafkaSchemaRef to KafkaSchema.Spec.References[] so a dependent KafkaSchema can resolve subject and version from another KafkaSchema in the same namespace
• Dependents auto-propagate when the referent advances: a controller-runtime field index + watch + predicate re-enqueues dependents on referent Status.Version changes.
• Delete is now a two-step soft + hard delete on the registry subject. Re-applying a KafkaSchema with the same subjectName after deletion starts cleanly at version 1
• Self-reference (A → A) blocked at admission
• Spec.References now uses +listType=map keyed by name, with MaxItems=100. Reference name uniqueness is enforced by the API server.
• kafkaSchemaVersionChangedPredicate filters watch events to referent moves, avoiding noisy re-enqueues

Resolves: NEX-2501 & #1149

[vmyroslav]

/test

[github-actions]

✅ Acceptance Tests Passed

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