Disabled validators runtime API (#1257)

Exposes disabled validators list via a runtime API.

---------

Co-authored-by: ordian <noreply@reusable.software>
Co-authored-by: ordian <write@reusable.software>

cumulus/client/relay-chain-rpc-interface/src/rpc_client.rs

@@ -597,6 +597,14 @@ impl RelayChainRpcClient {
 			.await
 	}
 
+	pub async fn parachain_host_disabled_validators(
+		&self,
+		at: RelayHash,
+	) -> Result<Vec<ValidatorIndex>, RelayChainError> {
+		self.call_remote_runtime_function("ParachainHost_disabled_validators", at, None::<()>)
+			.await
+	}
+
 	#[allow(missing_docs)]
 	pub async fn parachain_host_async_backing_params(
 		&self,