Broker: sale price runtime api (#3485)

Defines a runtime api for `pallet-broker` for getting the current price
of a core if there is an ongoing sale.

Closes: #3413

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/broker/src/runtime_api.rs

@@ -0,0 +1,31 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Runtime API definition for the FRAME Broker pallet.
+
+use codec::Codec;
+use sp_runtime::DispatchError;
+
+sp_api::decl_runtime_apis! {
+	pub trait BrokerApi<Balance>
+	where
+		Balance: Codec
+	{
+		/// If there is an ongoing sale returns the current price of a core.
+		fn sale_price() -> Result<Balance, DispatchError>;
+	}
+}