• Fixes Spring Kafka does not allow multiple headers with same key #3067

I know that there is an existing PR(#3101) related to this issue, but it seems to have been suspended for long time.
Therefore, I created a new PR to address it.
If you are planning to maintain the existing one, I am happy to close mine.

I approached the problem differently from the existing PR and proposed an alternative solution.
For backward compatibility, user can configure MultiValueKafkaHeaderMapper to KafkaListenerContainerFactory.
For example,

ConcurrentKafkaListenerContainerFactory<Integer, String> factory =
					new ConcurrentKafkaListenerContainerFactory<>();

MultiValueKafkaHeaderMapper headerMapper = new MultiValueKafkaHeaderMapper();

// Create new header mapper.
headerMapper.addSingleValueHeader(SINGLE_VALUE_HEADER);
MessagingMessageConverter converter = new MessagingMessageConverter(headerMapper);

// Set header mapper.
factory.setConsumerFactory(consumerFactory());