KIP-1157 — Enforce KafkaPrincipalSerde Implementation for KafkaPrincipalBuilder

Accepted Kafka 4.2 Security

Makes KafkaPrincipalBuilder extend KafkaPrincipalSerde, converting the need for custom principals to implement serialization into a compile-time requirement rather than a runtime failure. KRaft requires brokers to forward requests to controllers with serialized principal objects; without KafkaPrincipalSerde, custom KafkaPrincipalBuilder implementations silently failed at runtime during request forwarding.

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