Provide a `CancelOnGracefulShutdownService`

[fabianfett]

If an adopter already has a reference type that implements a run() method it is currently impossible to support ServiceLifecycle, even though the implementation works easily:

extension MyExistingService: Service {}

The reason for this is, that ServiceLifecycle uses gracefulShutdown to signal shutdown instead of Task cancellation. That is the correct call. However it is impossible to add gracefulShutdown support to object that already expose a run() method. That's why im proposing that ServiceLifecycle offers a CancelOnGracefulShutdownService<Underlying: Service>.