KIP-519 — Make SSL context/engine configuration extensible

Accepted Kafka 2.6 Security

Introduces `SslEngineFactory` as a new pluggable interface that allows users to supply custom `SSLContext` and `SSLEngine` creation logic via `ssl.engine.factory.class`, replacing the existing `ssl.keystore.*`/`ssl.truststore.*` file-based defaults where needed. Organizations with strict InfoSec requirements (e.g., HSM-backed keys, custom certificate validation) could not plug custom SSL behavior without forking Kafka's SSL implementation.

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