Introduce `JacksonJsonMessageConverter.getObjectMapper()`

[artembilan]

The now deprecated MappingJackson2MessageConverter has convenient getObjectMapper() for extensions.
In fact, in Spring for Apache Kafka we have:

public class MappingJacksonParameterizedConverter extends MappingJackson2MessageConverter {

where we use that getObjectMapper() for its logic.

Natural evolution would be to migrate to new JacksonJsonMessageConverter but that one does not have respective getObjectMapper().

Related issue: spring-projects/spring-kafka#3930

Thanks.

[sdeleuze]

I removed this getter when I removed the setter since we now use a final field + setting the ObjectMapper via constructor, but indeed I see how it can be useful, so I will restore it with protected access modifier.