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ARROW-12560: [C++] Add scheduling option for Future callbacks

Previously a future's callbacks would always run synchronously, either as part of `Future::MarkFinished` or as part of `Future::AddCallback`.  `Executor::Transfer` made it possible to schedule continuations on a new thread but it would only take effect if the transferred future's callbacks were added before the source future finished.  There are times when the desired behavior is to spawn a new thread task even if the source future is finished already.

This PR adds three scheduling options:
* Never - The default (and existing) behavior, never spawn a new task
* IfUnfinished - Spawn a new task only if the future isn't already finished when the callback is added
* Always - Always spawn a new task, on both finished and unfinished futures, regardless of destination thread pool idleness.

The `Never` option doesn't make any sense for transferring so the transfer only has two choices (always or if unfinished).

Closes #10258 from westonpace/feature/ARROW-12560--c-investigate-utilizing-aggressive-thread-task

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>