Support Client initiated Cancellation Propagation

Linking to the discussion, which contains the meat of the issue.

https://github.com/actix/actix-web/discussions/3011

Likewise, I'm aware of the difficult nature of this ask given the statelessness of the http protocol...

I believe Go handles this by polling the connection status internally and watching for CLOSE so that these events are propagated up.

Given actix-web positions itself as a higher level abstraction, we should also be able to have these events propagated up or that the very least the green-thread/connection needs to be exposed such that user can poll themselves.

Read the linked discussion #3011 first, since the issue says it contains the substantive requirements. Identify the relevant Actix Web request or connection entry points from that discussion and define done as client disconnect or cancellation being propagated upward, or the connection being exposed for polling; the payload names no files or tests.

Vom Indexierungsmodell aus dem Issue-Text verfasst.