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")))
```

cumulus/client/relay-chain-minimal-node/src/collator_overseer.rs

@@ -24,6 +24,7 @@ use polkadot_network_bridge::{
 	NetworkBridgeTx as NetworkBridgeTxSubsystem,
 };
 use polkadot_node_collation_generation::CollationGenerationSubsystem;
+use polkadot_node_core_prospective_parachains::ProspectiveParachainsSubsystem;
 use polkadot_node_core_runtime_api::RuntimeApiSubsystem;
 use polkadot_node_network_protocol::{
 	peer_set::PeerSetProtocolNames,
@@ -144,7 +145,7 @@ fn build_overseer(
 			spawner.clone(),
 		))
 		.statement_distribution(DummySubsystem)
-		.prospective_parachains(DummySubsystem)
+		.prospective_parachains(ProspectiveParachainsSubsystem::new(Metrics::register(registry)?))
 		.approval_distribution(DummySubsystem)
 		.approval_voting(DummySubsystem)
 		.gossip_support(DummySubsystem)

cumulus/client/relay-chain-minimal-node/src/lib.rs

@@ -171,12 +171,7 @@ async fn new_minimal_relay_chain(
 		);
 	}
 
-	let genesis_hash = relay_chain_rpc_client
-		.block_get_hash(Some(0))
-		.await
-		.expect("Genesis block hash is always available; qed")
-		.unwrap_or_default();
-
+	let genesis_hash = relay_chain_rpc_client.block_get_hash(Some(0)).await?.unwrap_or_default();
 	let peer_set_protocol_names =
 		PeerSetProtocolNames::new(genesis_hash, config.chain_spec.fork_id());
 	let is_authority = if role.is_authority() { IsAuthority::Yes } else { IsAuthority::No };