Cancel long poll if a Future returned from getResultAsync is not referenced anymore by the user code

Is your feature request related to a problem? Please describe.

If we hit WorkflowStub#getResultAsync and after that, we either don't wait on the result at all or wait for a bit and discard after, the long poll continues to run.
This creates a need for an unpleasant, unconventional WorkflowStub#getResultAsync(resultType, timeout).
Why do users need to pass a timeout to getResultAsync if they pass it into Future.get(timeout).
Most users just use WorkflowStub#getResultAsync(resultType) not understanding that they create a leak if the workflows don't finish fast.

Describe the solution you'd like
Instead of that, Temporal Java SDK code should maintain a weak or phantom reference on the CompletableFuture that we return to the users and cancel the background long poll when it's discarded by the user code.

Start at WorkflowStub#getResultAsync and trace how its CompletableFuture is connected to the background long poll. Determine how discarded futures can be detected safely and how the poll is cancelled, then verify that an unreferenced future no longer leaves a long poll running while a referenced future still completes normally.