Enable async backing on asset-hub-rococo (#2826)

The goal is to move all system chains on Rococo (+ other testnets?) to
use async backing.

Starting with `asset-hub-rococo` to get feedback, before I do the rest.

## Related

Example: https://github.com/paritytech/polkadot-sdk/pull/1619/
Guide:
https://github.com/w3f/polkadot-wiki/blob/master/docs/maintain/maintain-guides-async-backing.md

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>

Cargo.lock

@@ -864,6 +864,7 @@ dependencies = [
  "cumulus-pallet-session-benchmarking",
  "cumulus-pallet-xcm",
  "cumulus-pallet-xcmp-queue",
+ "cumulus-primitives-aura",
  "cumulus-primitives-core",
  "cumulus-primitives-utility",
  "frame-benchmarking",