KIP-189 — Improve principal builder interface and add support for SASL

Accepted Kafka 1.0 Security

Replaces the existing `PrincipalBuilder` interface (which only supports SSL) with an extended `KafkaPrincipalBuilder` interface that provides typed callbacks for both SSL (`SslAuthenticationContext`) and SASL (`SaslAuthenticationContext`) authentication contexts, enabling custom principal extraction from SASL credentials. The old interface had no SASL support and relied on `TransportLayer`/`Authenticator` internals that are not stable public API.

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