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ARROW-12004: [C++] Result<detail::Empty> is annoying

Per the JIRA

`Future<>::AddCallback` callbacks receive a `Status`.
`Future<T>::AddCallback` callbacks receive a `Result<T>`
`Future<>::Then` callbacks receive nothing
`Future<T>::Then` callbacks receive `const T&`

To achieve this I had to explicitly specialize the empty `Future` but I introduced `FutureBase` to reduce the amount of duplicated code.  `detail::Empty` is still around (although it got renamed to `internal::Empty` as a side effect of moving into `functional.h`).  It could even be removed if one wanted to create a specialized `FutureImpl` but that doesn't seem to be needed at the moment.

Closes #10205 from westonpace/feature/ARROW-12004--c-result-detail-empty-is-annoying

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>