Use the Chunk API consistently

This commit replaces the usage of List with Chunk
where appropriate. Summary of changes:

- The Chunk class was moved from the `org.springframework.batch.core.step.item` package to the `org.springframework.batch.item` package
- The signature of the method `ItemWriter#write(List)` was changed to `ItemWriter#write(Chunk)`
- All implementations of `ItemWriter` were updated to use the Chunk API instead of List
- All methods in the `ItemWriteListener` interface were updated to use the Chunk API instead of List
- All implementations of `ItemWriteListener` were updated to use the Chunk API instead of List
- The constructor of `ChunkRequest` was changed to accept a Chunk instead of a Collection of items
- The return type of `ChunkRequest#getItems()` was changed from List to Chunk

Resolves #3954

spring-batch-test/src/test/java/org/springframework/batch/test/StepScopeAnnotatedListenerIntegrationTests.java

@@ -34,6 +34,7 @@ import org.springframework.batch.core.configuration.annotation.EnableBatchProces
 import org.springframework.batch.core.configuration.annotation.JobBuilderFactory;
 import org.springframework.batch.core.configuration.annotation.StepBuilderFactory;
 import org.springframework.batch.core.configuration.annotation.StepScope;
+import org.springframework.batch.item.Chunk;
 import org.springframework.batch.item.ItemProcessor;
 import org.springframework.batch.item.ItemReader;
 import org.springframework.batch.item.ItemWriter;
@@ -144,7 +145,7 @@ class StepScopeAnnotatedListenerIntegrationTests {
 			return new ItemWriter<String>() {
 
 				@Override
-				public void write(List<? extends String> items) throws Exception {
+				public void write(Chunk<? extends String> items) throws Exception {
 				}
 			};
 		}