[0pt] Add cancellation support to ThreadPool

[ivan-cukic]

Make the following use-case work:

TEST_CASE("ThreadPool: Cancellation", "[ThreadPool][Cancellation]") {
    std::atomic<int>                            counter{ 0 };
    opencmw::BasicThreadPool<opencmw::IO_BOUND> pool("contention", 1, 4);
    pool.waitUntilInitialised();
    pool.execute(
            [&counter](std::stop_token stop_token) {
                while (!stop_token.stop_requested()) {
                    std::this_thread::sleep_for(std::chrono::milliseconds(10));
                }
            });
}

This will allow interrupting running threads when the application wants to shut down.

[RalphSteinhagen]

done and implemented by @wirew0rm in

• Thread pool gnuradio4#67
• scheduler: add thread pool backed parallel implementation to "simple" and "breadth_first" schedulers gnuradio4#78