Additional transport costs in XCM bridge

[svyatonik]

related to #1318 and #1417

So in encoded calls messaging the total message_delivery_and_dispatch_fee had three main components:

1. cost of message delivery transaction;
2. cost of message dispatch;
3. cost of delivery confirmation transaction.

With XCM coming, iiuc there's an additional problem - our (bridge target) chain may not match the message target chain (where message needs to be dispatched. For example - we're sending message from Millau to RialtoParachain, but the message needs to be dispatched at the Rialto itself, so we (XCM executor/router?) need to use UMP (?) to deliver message to Rialto.

The question is: who's paying cost of this UMP transfer? Is it handled by the XCM executor by some instruction? Or we need to withdraw this fee from the message sender at the source chain? I don't have idea right now and it needs to be explored.

[svyatonik]

The fee will be paid at the source chain (not at bridge hub as it has been designed before). It shall cover all costs. The issue is not relevant anymore