[pallet-xcm] fix transport fees for remote reserve transfers (#3792)

Currently `transfer_assets` from pallet-xcm covers 4 main different
transfer types:
- `localReserve`
- `DestinationReserve`
- `Teleport`
- `RemoteReserve`

For the first three, the local execution and the remote message sending
are separated, and fees are deducted in pallet-xcm itself:
https://github.com/paritytech/polkadot-sdk/blob/3410dfb3929462da88be2da813f121d8b1cf46b3/polkadot/xcm/pallet-xcm/src/lib.rs#L1758.

For the 4th case `RemoteReserve`, pallet-xcm is still relying on the
xcm-executor itself to send the message (through the
`initiateReserveWithdraw` instruction). In this case, if delivery fees
need to be charged, it is not possible to do so because the
`jit_withdraw` mode has not being set.

This PR proposes to still use the `initiateReserveWithdraw` but
prepending a `setFeesMode { jit_withdraw: true }` to make sure delivery
fees can be paid.

A test-case is also added to present the aforementioned case

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

prdoc/pr_3792.prdoc

@@ -0,0 +1,19 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "[pallet-xcm] fix transport fees for remote reserve transfers"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR fixes `pallet_xcm::transfer_assets` and
+      `pallet_xcm::limited_reserve_transfer_assets` extrinsics for transfers
+      that need to go through remote reserves. The fix is adding a
+      `SetFeesMode { jit_withdraw: true }` instruction before local execution of
+      `InitiateReserveWithdraw` so that delivery fees are correctly charged by
+      the xcm-executor. Without this change, a runtime that has implemented
+      delivery fees would not be able to execute remote reserve transfers using
+      these extrinsics.
+
+crates:
+  - name: pallet-xcm