Update UI to reflect DMP/HRMP message delivery fees

[KiChjang]

After paritytech/polkadot#6843 and paritytech/polkadot#7005 have been merged, we will need to look for a place to inform the user of the delivery fees to collect for the XCM executor to send an outgoing XCM.

This new delivery fee is not included in the weight of the extrinsic and is collected separately. The formula for calculating the delivery fee is:

delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee)

The base_fee is a set constant in every chain, and is defined as CENTS * 3. The encoded_msg_len can be determined simply by encoding the XCM and checking the resulting encoded blob size in bytes, and the per_byte_fee is just the TransactionByteFee which is defined as 10 * MILLICENTS on every chain.

The only weird one is delivery_fee_factor, but that is also easily fetched by checking the chain state on the DMP pallet (the XCMP queue pallet if on HRMP) via the DeliveryFeeFactor storage map. This map is keyed on the destination's ParaId, and if no entries exist yet for the given parachain, then the delivery fee factor would just be 1.

[jacogr]

Umpf, this is slightly problematic since it means for each chain we will need to know what CENTS refers to.

Is there no way to expose this as a runtime API, meaning that if any changes is made on the chain we don't maintain this in different places? This is really what is done with fee calculations, instead of hardcoding there is an API which means that the whole world doesn't need to keep track of how calculations are done.

I may not be that bad if all those above are exposed as constants in the metadata, but we obviously still need adjustment if the formula changes, i.e. needs to be kept track of. (It does look pretty stock-standard through, it is always base + len * fee in the past)

[xlc]

I also wrote this issue but no one cares 😢 paritytech/polkadot-sdk#690 paritytech/cumulus#2342 (comment)

[KiChjang]

I don't believe the formula is ever going to change, but the constants may, as it has happened with Kusama. I think as long as we have a runtime API to keep track of CENTS and MILLICENTS, then we're good. Do we?

[jacogr]

Never say never - trust me on this one…