feat: Extract generic type for KafkaListeners in batch mode

[timonback]

The KafkaPayloadTypeExtractor contains the logic to extract the payload type, which previously was part of ClassLevelKafkaListenerScanner and MethodLevelKafkaListenerScanner.

If the parameter is of type List<?>, reflection is used to extract the underlying type.
This solution assumes by using the List type, Kafka batch mode is used.

A future improvement can be to only extract the generic when the KafkaListener annotation is used together with the batch property. A library update for spring-kafka is possible: https://docs.spring.io/spring-kafka/api/org/springframework/kafka/annotation/KafkaListener.html#batch()

To demo it, one listener in ExampleConsumer is updated to use batch mode.

Relates to #87

[stavshamir]

Great work, it's good to see so much code re-used. Please see my comments, I really think we can use this extractor for the amqp plugin as well

[timonback]

I updated the PR with a larger refactoring and moving duplicated code to Abstract classes in core. The amqp and kafka listeners are slimmer now, although there is still some complexity through the abstract methods.
For reviewing, I suggest to check out the code locally.

Since the release is around the corner, I think this is a feature for the following release.

Regarding the batch mode, yes, this by default assumes that all List Payloads are batch methods. In the future this could be made configurable (batch-mode: yes/no), but at this point I would assume that people define their own payload types and not use List in non-batch mode (defining <MyType extends List<?> should work around the list detection anyway).

[stavshamir]

I really like this refactoring, it will make it much much easier to implement more plugins based on listener annotations. Great job!