Add Kafka ItemWriter

This commit adds an ItemWriter implementation for Apache Kafka using
a KafkaTemplate with default topic set. Extending KeyValueItemWriter,
the writer requires a Converter to map the Kafka ProducerRecord's key
from the item.

Resolves BATCH-2765

spring-batch-docs/asciidoc/appendix.adoc

@@ -102,6 +102,9 @@
             and handles some transaction-related work that a non-"JPA-aware"
             `ItemWriter` would not need to know about and
             then delegates to another writer to do the actual writing.
+|KafkaItemWriter|Using a `KafkaTemplate` object, items are written to the default topic through the
+            `KafkaTemplate#sendDefault(Object, Object)` method using a `Converter` to map the key from the item.
+            A delete flag can also be configured to send delete events to the topic.
 |MimeMessageItemWriter|Using Spring's `JavaMailSender`, items of type `MimeMessage`
             are sent as mail messages.
 |MongoItemWriter|Given a `MongoOperations` object, items are written

spring-batch-docs/asciidoc/readersAndWriters.adoc

@@ -3054,6 +3054,7 @@ Spring Batch offers the following readers and writers for commonly used messagin
 * <<amqpItemWriter>>
 * <<jmsItemReader>>
 * <<jmsItemWriter>>
+* <<kafkaItemWriter>>
 
 [[amqpItemReader]]
 ===== `AmqpItemReader`
@@ -3081,6 +3082,12 @@ The `JmsItemWriter` is an `ItemWriter` for JMS that uses a `JmsTemplate`. The te
 should have a default destination, which is used to send items in `write(List)`. Spring
 Batch provides a `JmsItemWriterBuilder` to construct an instance of the `JmsItemWriter`.
 
+[[kafkaItemWriter]]
+===== `KafkaItemWriter`
+The `KafkaItemWriter` is an `ItemWriter` for Apache Kafka that uses a `KafkaTemplate` to
+send events to a default topic. Spring Batch provides a `KafkaItemWriterBuilder` to
+construct an instance of the `KafkaItemWriter`.
+
 [[databaseReaders]]
 ==== Database Readers
 Spring Batch offers the following database readers: