DLQ - properly shutdown the flusher scheduled service used by DLQ writer

[andsel]

Add shutdown to SchedulerService and invoke in DLQ writer close operation.

Adding a clean shutdown of the service used to flush the DLQ segments, during test avoid printing to stdout errors like:

org.logstash.config.ir.compiler.DatasetCompilerTest > compilesOutputDataset STANDARD_OUT
    [WARN ] 2024-02-21 12:46:44.723 [dlq-flush-check] DeadLetterQueueWriter - Unable to finalize segment
    java.nio.file.NoSuchFileException: /var/folders/f2/6ln9srr13hsdp3kwfz68w3940000gn/T/junit3045222654236224377/junit12477847196935554659/2.log.tmp -> /var/folders/f2/6ln9srr13hsdp3kwfz68w3940000gn/T/junit3045222654236224377/junit12477847196935554659/2.log
        at sun.nio.fs.UnixException.translateToIOException(UnixException.java:92) ~[?:?]
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106) ~[?:?]
        at sun.nio.fs.UnixCopyFile.move(UnixCopyFile.java:416) ~[?:?]
        at sun.nio.fs.UnixFileSystemProvider.move(UnixFileSystemProvider.java:266) ~[?:?]
        at java.nio.file.Files.move(Files.java:1432) ~[?:?]
        at org.logstash.common.io.DeadLetterQueueWriter.sealSegment(DeadLetterQueueWriter.java:586) ~[main/:?]
        at org.logstash.common.io.DeadLetterQueueWriter.finalizeSegment(DeadLetterQueueWriter.java:572) ~[main/:?]
        at org.logstash.common.io.DeadLetterQueueWriter.scheduledFlushCheck(DeadLetterQueueWriter.java:543) ~[main/:?]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539) [?:?]
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:305) [?:?]
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:305) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
        at java.lang.Thread.run(Thread.java:840) [?:?]

org.logstash.config.ir.compiler.DatasetCompilerTest > compilesOutputDataset PASSED