GH-5185: Move async processing APIs to core

[won-seoop]

Closes #5185

This pull request moves async item processing and local chunking support into spring-batch-core:

• Adds AsyncItemProcessor, AsyncItemWriter, and ChunkTaskExecutorItemWriter to spring-batch-core
• Keeps the existing spring-batch-integration types as deprecated wrappers for source compatibility
• Updates samples and documentation to use the core package
• Adds unit tests for the moved core types

Validation:

• ./mvnw -pl spring-batch-core -Dtest=AsyncItemProcessorTests,AsyncItemWriterTests,ChunkTaskExecutorItemWriterTests test
• ./mvnw -pl spring-batch-integration,spring-batch-samples -am -DskipTests compile

Note: local validation was run on JDK 21 with temporary compiler settings for NullAway JSpecify support; those local settings are not part of this pull request.

[pkernevez]

I put the comment here as I don't really know where to put it as the class is on transit.
There is an existing bug here or at least an issue with the documentation.
When starting a new step we have to reset the state too:

Suggested change

	this.stepExecution = stepExecution;
	this.stepExecution = stepExecution;
	this.responses.clear();
	this.sequence = 0;

Without this, we mixe the state of the previous runs.
Or in the documentation show an example with a Scope more volatile than a singleton:

@Bean
public ChunkTaskExecutorItemWriter<Vet> itemWriter(ChunkProcessor<Vet> chunkProcessor) {
    ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
    taskExecutor.setCorePoolSize(4);
    taskExecutor.setThreadNamePrefix("worker-thread-");
    taskExecutor.setWaitForTasksToCompleteOnShutdown(true);
    taskExecutor.afterPropertiesSet();
    return new ChunkTaskExecutorItemWriter<>(chunkProcessor, taskExecutor);
}