public SelectionKey selectionKey() {
        return transportLayer.selectionKey();
    }
Does handshake of transportLayer and authentication using configured authenticator.
For SSL with client authentication enabled, {@link TransportLayer#handshake()} performs
authentication. For SASL, authentication is performed by {@link Authenticator#authenticate()}.
clients/src/main/java/org/apache/kafka/common/network/KafkaChannel.java