Add a way to gracefully stop all children actors in the supervision tree recursively

[contrun]

Is your feature request related to a problem? Please describe.
Supervision tree can be a particularly useful tool for resource management, as it potentially can reap all the resources of the children actors. But currently, when the parent actor is stopped, all the children are killed without any opportunity to run post_stop. This makes it impossible for them to release all the resources.

Describe the solution you'd like
Add a function to stop all the actors in the tree and run their post_stop functions. This should preferably run in a recursive way (or at least have such function to run it recursively).

Describe alternatives you've considered
Manual management is boring.

Additional context
post_stop of children are being called when supervisor fails. · Issue #226 · slawlor/ractor