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>

cumulus/parachains/runtimes/coretime/coretime-westend/src/lib.rs

@@ -64,7 +64,7 @@ use sp_runtime::{
 	create_runtime_str, generic, impl_opaque_keys,
 	traits::Block as BlockT,
 	transaction_validity::{TransactionSource, TransactionValidity},
-	ApplyExtrinsicResult, MultiAddress, Perbill,
+	ApplyExtrinsicResult, DispatchError, MultiAddress, Perbill,
 };
 use sp_std::prelude::*;
 #[cfg(feature = "std")]
@@ -584,6 +584,12 @@ impl_runtime_apis! {
 		}
 	}
 
+	impl pallet_broker::runtime_api::BrokerApi<Block, Balance> for Runtime {
+		fn sale_price() -> Result<Balance, DispatchError> {
+			Broker::current_price()
+		}
+	}
+
 	impl pallet_transaction_payment_rpc_runtime_api::TransactionPaymentApi<Block, Balance> for Runtime {
 		fn query_info(
 			uxt: <Block as BlockT>::Extrinsic,