* Reintrouce BinderHeaderMapper
Provide a custom header mapper that is identical to the DefaultKafkaHeaderMapper in Spring Kafka.
This is to address some interoperability issues between Spring Cloud Stream 3.0.x and 2.x apps,
where mime types in the header are not de-serialized properly. This custom BinderHeaderMapper
will be eventually deprecated and removed once the fixes are in Spring Kafka.
Resolves#796
* Addressing review
* polishing
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