# Description

Sending XCM messages to other chains requires paying a "transport fee".
This can be paid either:
- from `origin` local account if `jit_withdraw = true`,
- taken from Holding register otherwise.

This currently works for following hops/scenarios:
1. On destination no transport fee needed (only sending costs, not
receiving),
2. Local/originating chain: just set JIT=true and fee will be paid from
signed account,
3. Intermediary hops - only if intermediary is acting as reserve between
two untrusted chains (aka only for `DepositReserveAsset` instruction) -
this was fixed in https://github.com/paritytech/polkadot-sdk/pull/3142

But now we're seeing more complex asset transfers that are mixing
reserve transfers with teleports depending on the involved chains.

# Example

E.g. transferring DOT between Relay and parachain, but through AH (using
AH instead of the Relay chain as parachain's DOT reserve).

In the `Parachain --1--> AssetHub --2--> Relay` scenario, DOT has to be
reserve-withdrawn in leg `1`, then teleported in leg `2`.
On the intermediary hop (AssetHub), `InitiateTeleport` fails to send
onward message because of missing transport fees. We also can't rely on
`jit_withdraw` because the original origin is lost on the way, and even
if it weren't we can't rely on the user having funded accounts on each
hop along the way.

# Solution/Changes

- Charge the transport fee in the executor from the transferred assets
(if available),
- Only charge from transferred assets if JIT_WITHDRAW was not set,
- Only charge from transferred assets if unless using XCMv5 `PayFees`
where we do not have this problem.

# Testing

Added regression tests in emulated transfers.

Fixes https://github.com/paritytech/polkadot-sdk/issues/4832
Fixes https://github.com/paritytech/polkadot-sdk/issues/6637

---------

Signed-off-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>