[C++] Allow futures to be added as tasks to a TaskGroup

[asfimport]

Once basic continuation support is in, if we can add futures to a task group, and a task group completion can be expressed as a future, then we can use task groups in such a way that they will not deadlock.

This task focuses on the ability to add a future to a task group instead of spawning a new task.

Reporter: Weston Pace / @westonpace

_{Note: This issue was originally created as ARROW-10184. Please see the migration documentation for further details.}