Missing `@FunctionalInterface` in JobKeyGenerator

[acktsap]

Since #3926 (pr), it's been a public api for spring batch configuration. But there is no @FunctionalInterface in the api.

I think it should be added to the api.

spring-batch/spring-batch-core/src/main/java/org/springframework/batch/core/JobKeyGenerator.java

Lines 18 to 37 in 425134c

	/**
	* Strategy interface for the generation of the key used in identifying unique
	* {@link JobInstance} objects.
	*
	* @author Michael Minella
	* @author Mahmoud Ben Hassine
	* @param <T> The type of the source data used to calculate the key.
	* @since 2.2
	*/
	public interface JobKeyGenerator<T> {
	/**
	* Method to generate the unique key used to identify a job instance.
	* @param source Source information used to generate the key (must not be
	* {@code null}).
	* @return a unique string identifying the job based on the information supplied.
	*/
	String generateKey(T source);
	}