don't depend on BootstrapRuntime, use an akka-extension instead

[domdorn]

The current traits depend on BootstrapRuntime. This makes testing hard.

I suggest an alternative approach:

1. Create a Akka-Extension, e.g. ZIOAkkaHttp that stores the Runtime. This one can be registered on application startup, as soon as an ActorSystem is available.
2. Adjust the effects to work as a directive. There is already the extractActorSystem directive, so we'd be able to do something like:

extractActorSystem( ac => ZIOAkkaHttp(ac).getRuntime.unsafeRunToFuture(f))

What do you think?