DeliveryConfirmationPayments trait for paying relayer rewards at the source chain (#1653)

* DeliveryConfirmationPayments

* ref proper PR in TODO

bridges/primitives/messages/src/source_chain.rs

@@ -125,6 +125,37 @@ impl<SenderOrigin, AccountId> MessageDeliveryAndDispatchPayment<SenderOrigin, Ac
 	}
 }
 
+/// Manages payments that are happening at the source chain during delivery confirmation
+/// transaction.
+pub trait DeliveryConfirmationPayments<AccountId> {
+	/// Error type.
+	type Error: Debug + Into<&'static str>;
+
+	/// Pay rewards for delivering messages to the given relayers.
+	///
+	/// The implementation may also choose to pay reward to the `confirmation_relayer`, which is
+	/// a relayer that has submitted delivery confirmation transaction.
+	fn pay_reward(
+		lane_id: LaneId,
+		messages_relayers: VecDeque<UnrewardedRelayer<AccountId>>,
+		confirmation_relayer: &AccountId,
+		received_range: &RangeInclusive<MessageNonce>,
+	);
+}
+
+impl<AccountId> DeliveryConfirmationPayments<AccountId> for () {
+	type Error = &'static str;
+
+	fn pay_reward(
+		_lane_id: LaneId,
+		_messages_relayers: VecDeque<UnrewardedRelayer<AccountId>>,
+		_confirmation_relayer: &AccountId,
+		_received_range: &RangeInclusive<MessageNonce>,
+	) {
+		// this implementation is not rewarding relayers at all
+	}
+}
+
 /// Send message artifacts.
 #[derive(Eq, RuntimeDebug, PartialEq)]
 pub struct SendMessageArtifacts {