feat: add QueueClosed singleton instance for Java API

[Roiocam]

Motivation

new API for Java users, used to detect queue completion.

QueueOfferResult offerResult = queue.offer(params);

if (offerResult == QueueOfferResult.enqueued()) {
    // logic...
} else if (offerResult == QueueOfferResult.dropped()) {
    // logic...
} else if (offerResult instanceof QueueOfferResult.Failure) {
    // logic...
} else if (offerResult == QueueOfferResult.closed()) {
    // logic...
} else {
    // logic...
}

pekko/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/BoundedSourceQueueSpec.scala

Lines 98 to 106 in 3fabc04

	"return a QueueClosed result if completed" in {
	val sub = TestSubscriber.probe[Int]()
	val queue =
	Source.queue[Int](10).toMat(Sink.fromSubscriber(sub))(Keep.left).run()
	queue.complete()
	queue.isCompleted shouldBe true
	queue.offer(1) should be(QueueOfferResult.QueueClosed)
	sub.expectSubscriptionAndComplete()

[He-Pin]

You can use isEqueued too.