Prepare serve for potentially supporting graceful shutdown

[davidpdrsn]

I agree that graceful shutdown is probably the most common thing missing from axum::serve and its not exactly easy with the current state of hyper-util (see #2355).

So I could probably convinced to add graceful shutdown support to serve 😅 I think a good API for that would be

axum::serve(listener, service)
    .graceful_shutdown(shutdown_signal_future())
    .await

Doing that requires returning some type we control from serve but we can use IntoFuture so you can still do serve(...).await.

This needs to go in before shipping 0.7 since its a breaking change.

[jplatte]

Good idea to prepare for the possibility without blocking the release on actually adding it!