Job: implement onFinish()

It's useful to do some processing after job have finished.

Currently you can use custom `Executor` to accomplish this but that's not as simple and easy as:
```cpp
auto job = ThreadWeaver::make_job([]() {
    // Job Lambda
});

job->onFinish([](const ThreadWeaver::JobInterface &job) {
    // Finish handler Lambda
});

ThreadWeaver::enqueue(job);
```

This is most useful for `Collection`/`Sequence` because for those you want handler once all jobs have finished not after each of them.