Update DefaultRocketMQListenerContainer#getMessageType to protected

[ruansheng8]

What is the purpose of the change

public interface IMessageConsumer<T> extends RocketMQListener<T> {
}

Due to project needs, some personalized business operations need to extend the RocketMQListener interface.
I need to override the DefaultRocketMQListenerContainer#getMessageType() method to complete my custom extension.

[ruansheng8]

@RongtongJin cc