multi: Fix final hop payload size for blinded rt.

The final hop size is calculated differently therefore we extract
the logic in its own function and also account for the case where
the final hop might be a blinded hop.

lnrpc/routerrpc/router_backend.go

@@ -390,6 +390,7 @@ func (r *RouterBackend) parseQueryRoutesRequest(in *lnrpc.QueryRoutesRequest) (
 		DestCustomRecords: record.CustomSet(in.DestCustomRecords),
 		CltvLimit:         cltvLimit,
 		DestFeatures:      destinationFeatures,
+		BlindedPayment:    blindedPmt,
 	}
 
 	// Pass along an outgoing channel restriction if specified.