KIP-44 — Allow Kafka to have a customized security protocol

Discussion Security

KIP-44 introduces a `CUSTOMIZED` security protocol and a `customized.network.channelbuilder` config that accepts a user-provided `ChannelBuilder` implementation, enabling arbitrary authentication and transport mechanisms beyond Kerberos and SSL. This allows users with existing token-based or non-GSSAPI authentication systems to integrate natively without rebuilding Kafka.

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