Support Client initiated Cancellation Propagation

[marcsantiago]

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.