[C++] Simplify or remove `arrow/util/functional.h`

[pitrou]

Describe the enhancement requested

arrow/util/functional.h is a set of ad-hoc metaprogramming utilities that were necessary when Arrow supported C++11. Now that we require C++20, most of that functionality can probably be replaced by stdlib utilites such as std::is_invocable and std::invoke_result.

This issue requires some C++ proficiency but hopefully little knowledge of the Arrow C++ codebase.

Component(s)

C++

[pitrou]

cc @tadeja @taepper FYI

[HuaHuaY]

Is someone working on this issue? I have refactored part of this file in iceberg-cpp repo https://github.com/apache/iceberg-cpp/blob/main/src/iceberg/util/functional.h. If there is no one working on this, I'd like to fix this issue.

[tadeja]

@HuaHuaY I'm not working on this issue yet.
@taepper have you perhaps started this? (I see you're on #50248 right now:)

[taepper]

I have not started working on this :)