public SaslExtensions extensions() {
        return extensions;
    }
Returns always non-null {@link SaslExtensions} consisting of the extension
names and values that are sent by the client to the server in the initial
client SASL authentication message. The default value is
{@link SaslExtensions#empty()} so that if this callback is
unhandled the client will see a non-null value.
clients/src/main/java/org/apache/kafka/common/security/auth/SaslExtensionsCallback.java