[PORT] Akka/Akka#27266 - Propagate stream cancellation causes

[Arkatufus]

Port of akka/akka#27266 and akka/akka#27547

From the original PR:

This PR is focussed on adding APIs and machinery to provide cancellation causes across stages and reactive streams interfaces. The new APIs are (so far) completely optional:

• A reactive streams subscription can implement SubscriptionWithCancelException if it wants to support that feature
• There's a new GraphStage.cancel(cause) method that can be used to provide a cause (otherwise, SubscriptionWithCancelException.NoMoreElementsNeeded is used)
• There's a new OutHandler.onDownstreamFinished(cause) method to override if you want access to (or to propagate) the cancellation cause. If not overridden it calls the old method.
• Similar APIs (SubSink etc.) have already also been adapted.

These changes should be completely transparent and only noticeable to those parties that implement the APIs.

[Aaronontheweb]

LGTM

[Aaronontheweb]

@ismaelhamed I'll wait for your comment before merging it; hoping to get this pulled in sometime next week.

[ismaelhamed]

LGTM.

This felt like a punishment to review. Now I get why @Aaronontheweb sometimes complains about PRs too big ;)