Support subflow cancellation if parent flow is in terminal state about prefect HOT 2 OPEN

Thanks for the enhancement request @j-tr! One question about your setup: do you wait for these deployment runs to complete at any point in your parent flow?

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hello @desertaxle, we are usually waiting for the deployment runs to reach a final state.
i think there are at least two usecases:

1. the developer wants to start sub deployments asynchronously and returns from the parent flow before waiting for the subflows to finish. In this case, the current behavior is expected. However, group cancellation of all subflows could still be useful.
2. the developer is actually looking for sub flow behavior (flow run as python function, not via run_deployment) but the subflow requires a specific infrastructure that is only available via a different work pool. Therefore, it needs to run as a deployment. In order to get as close as possible to the sub-flow behavior, we use a polling mechanism like in https://github.com/PrefectHQ/prefect/blob/main/src/prefect/deployments/deployments.py#L230-L236 at the end of the parent flow to join all subflows and set the return value of the parent flow according to the states of the sub flows. In this case, it makes no sense to continue the subflows when the parent flow fails or crashes.

See #6689 for a related feature request.

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