Use the Chunk API consistently

[fmbenhassine]

A chunk of items is a key concept in the chunk-oriented processing model and is nicely modelled with the org.springframework.batch.core.step.item.Chunk API. This API is used in almost all interfaces/classes related to the implementation of this model (ie ChunkProvider, ChunkProcessor, etc) except for the ItemWriter<T> where a List<? extends T> is used instead. I believe Chunk<T> is a better encapsulation for the chunk of items than a List<? extends T>:

public interface ItemWriter<T> {
	
-- void write(List<? extends T> items) throws Exception;
++ void write(Chunk<? extends T> items) throws Exception;

}

This would make the ItemWriter API more coherent and consistent with the rest of the package. This is obviously a breaking change and requires moving the Chunk API to the infrastructure module, so it is a good candidate for v5 in my opinion.

NB: This should not be an issue for the JSR-352 implementation (which is deprecated anyway) as there is already an adapter for the item writer: org.springframework.batch.jsr.item.ItemWriterAdapter. This adapter can be updated to adapt a Chunk<T> to a List<Object> as expected by the JSR.

In the spring-batch-integration module, ChunkRequest could also be updated to use Chunk<T> instead of Collection<? extends T> for consistency.