feat: add on_exit hook

[ThomasCrambert]

Expose new on_exit hook to allow post-shutdown cleanup actions.

Notes:

• I made it for all kind of processes, but I believe it would only be useful for workers as they are the only one that may require such hooks. Do you believe it should only be for workers now and add them for the others later?
• Dispatchers currently trigger run_stop_hooks in after_shutdown but workers trigger them at before_shutdown, I changed it in this PR but was that the expected behavior?

[rosa]

Thanks @ThomasCrambert!

Dispatchers currently trigger run_stop_hooks in after_shutdown but workers trigger them at before_shutdown, I changed it in this PR but was that the expected behavior?

Oh! No, that was an oversight. Thanks for catching that 🙏

[rosa]

Do you believe it should only be for workers now and add them for the others later?

No, I think this is fine 👍