bevy_tasks: Drop static TaskPools or wait until all tasks are finished

## What problem does this solve or what need does it fill?

I'd like to make sure there's no more remaining work on the static task pools when exiting the application or just when freeing the engine.

## What solution would you like?

I'd like to be able to drop the static task pools such as the [ComputeTaskPool](https://docs.rs/bevy_tasks/latest/bevy_tasks/struct.ComputeTaskPool.html) to make sure their work is done and their threads are "joined".

It would be nice to have the option to:
- Wait until all queued work is done
- Cancel remaining work
- Drop the TaskPool and join the threads

## What alternative(s) have you considered?

Not all of the things I listed but just waiting for all queued work to be completed.