Add prospective-parachain subsystem to minimal-relay-node + QoL improvements (#2223)
This PR contains some fixes and cleanups for parachain nodes: 1. When using async backing, node no longer complains about being unable to reach the prospective-parachain subsystem. 2. Parachain warp sync now informs users that the finalized para block has been retrieved. ``` 2023-11-08 13:24:42 [Parachain]🎉 Received finalized parachain header #5747719 (0xa0aa…674b) from the relay chain. ``` 3. When a user supplied an invalid `--relay-chain-rpc-url`, we were crashing with a very verbose message. Removed the `expect` and improved the error message. ``` 2023-11-08 13:57:56 [Parachain] No valid RPC url found. Stopping RPC worker. 2023-11-08 13:57:56 [Parachain] Essential task `relay-chain-rpc-worker` failed. Shutting down service. Error: Service(Application(WorkerCommunicationError("RPC worker channel closed. This can hint and connectivity issues with the supplied RPC endpoints. Message: oneshot canceled"))) ```