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GH-35176: [C++] Add support for disabling threading for emscripten (#35672)

As previously discussed in #35176 this is a patch that adds an option `ARROW_ENABLE_THREADING`. When it is turned off, arrow threadpool and serial executors don't spawn threads, and instead run tasks in the main thread when futures are waited for.

It doesn't mess with threading in projects included as dependencies, e.g. multithreaded malloc implementations because if you're building for a non threaded environment, you can't use those anyway.

Basically where this is at is that it runs the test suite okay, and I think should work well enough to be a backend for pandas on emscripten/pyodide.

What this means is:
1) It is possible to use arrow in non-threaded emscripten/webassembly environments (with some build patches specific to emscripten which I'll put in once this is in)
2) Most of arrow just works, albeit slower in parts.

Things that don't work and probably won't:
1) Server stuff that relies on threads. Not a massive problem I think because environments with threading restrictions are currently typically also restricted from making servers anyway (i.e. they are web browsers)
2) Anything that relies on actually doing two things at once (for obvious reasons)

Things that don't work yet and could be fixed in future:
1) use of asynchronous file/network APIs in emscripten which would mean I/O could work efficiently in one thread.
2) asofjoin - right now the implementation relies on std::thread - it needs refactoring to work with threadpool like everything else in arrow, but I'm not sure I am expert enough in the codebase to do it well.
* Closes: #35176

Lead-authored-by: Joe Marshall <joe.marshall@nottingham.ac.uk>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>