Add OnMessageAccepted callback (#1134)

* Add `OnMessageAccepted` config type

* Update actual weight

* Add unit test

* Update weight

* Remove old comment & update wrong test data

* Make ci happy

* Add lane_id param

* update test case

* Make log info more readable

* Use saturating_sub

* Update docs

bridges/primitives/messages/src/source_chain.rs

@@ -173,6 +173,18 @@ impl OnDeliveryConfirmed for Tuple {
 	}
 }
 
+/// Handler for messages have been accepted
+pub trait OnMessageAccepted {
+	/// Called when a message has been accepted by message pallet.
+	fn on_messages_accepted(lane: &LaneId, message: &MessageNonce) -> Weight;
+}
+
+impl OnMessageAccepted for () {
+	fn on_messages_accepted(_lane: &LaneId, _message: &MessageNonce) -> Weight {
+		0
+	}
+}
+
 /// Structure that may be used in place of `TargetHeaderChain`, `LaneMessageVerifier` and
 /// `MessageDeliveryAndDispatchPayment` on chains, where outbound messages are forbidden.
 pub struct ForbidOutboundMessages;