Feature request: Add BoundedSourceQueue#isComplete method

[He-Pin]

Motivation:
I want to do a checking before enqueue or call complete with a check otherwise it will throw an exception .

@InternalApi private[pekko] object BoundedSourceQueueStage {
  sealed trait State
  case object NeedsActivation extends State
  case object Running extends State
  case class Done(result: QueueCompletionResult) extends State
}

Modification:
Add BoundedSourceQueue#isComplete method.

Cons:
But that will introduce another check, hurt performance for real usage.

Update:
Checked reactor-core too, which doesn't provides something like this too.