multi: introduce new traffic shaper method.

We introduce a new specific fail resolution error when the
external HTLC interceptor denies the incoming HTLC. Moreover
we introduce a new traffic shaper method which moves the
implementation of asset HTLC to the external layers.
Moreover itests are adopted to reflect this new change.

routing/bandwidth_test.go

@@ -164,3 +164,7 @@ func (*mockTrafficShaper) ProduceHtlcExtraData(totalAmount lnwire.MilliSatoshi,
 
 	return totalAmount, nil, nil
 }
+
+func (*mockTrafficShaper) IsCustomHTLC(_ lnwire.CustomRecords) bool {
+	return false
+}