KIP-473 — Enable KafkaLog4JAppender to use SASL Authentication Callback Handlers

Discussion SecurityClient

Extends `KafkaLog4JAppender` to support pluggable SASL authentication callback handlers via `sasl.login.callback.handler.class` and `sasl.client.callback.handler.class` properties, consistent with KIP-86. The existing `KafkaLog4JAppender` with SASL (added by KIP-425) requires credentials to be hardcoded in the JAAS configuration, preventing use of dynamic credential providers like AWS IAM or Vault for log appenders.

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