pallet-xcm: enhance reserve_transfer_assets to support remote reserves (#1672)

## Motivation

`pallet-xcm` is the main user-facing interface for XCM functionality,
including assets manipulation functions like `teleportAssets()` and
`reserve_transfer_assets()` calls.

While `teleportAsset()` works both ways, `reserve_transfer_assets()`
works only for sending reserve-based assets to a remote destination and
beneficiary when the reserve is the _local chain_.

## Solution

This PR enhances `pallet_xcm::(limited_)reserve_withdraw_assets` to
support transfers when reserves are other chains.
This will allow complete, **bi-directional** reserve-based asset
transfers user stories using `pallet-xcm`.

Enables following scenarios:
- transferring assets with local reserve (was previously supported iff
asset used as fee also had local reserve - now it works in all cases),
- transferring assets with reserve on destination,
- transferring assets with reserve on remote/third-party chain (iff
assets and fees have same remote reserve),
- transferring assets with reserve different than the reserve of the
asset to be used as fees - meaning can be used to transfer random asset
with local/dest reserve while using DOT for fees on all involved chains,
even if DOT local/dest reserve doesn't match asset reserve,
- transferring assets with any type of local/dest reserve while using
fees which can be teleported between involved chains.

All of the above is done by pallet inner logic without the user having
to specify which scenario/reserves/teleports/etc. The correct scenario
and corresponding XCM programs are identified, and respectively, built
automatically based on runtime configuration of trusted teleporters and
trusted reserves.

#### Current limitations:
- while `fees` and "non-fee" `assets` CAN have different reserves (or
fees CAN be teleported), the remaining "non-fee" `assets` CANNOT, among
themselves, have different reserve locations (this is also implicitly
enforced by `MAX_ASSETS_FOR_TRANSFER=2`, but this can be safely
increased in the future).
- `fees` and "non-fee" `assets` CANNOT have **different remote**
reserves (this could also be supported in the future, but adds even more
complexity while possibly not being worth it - we'll see what the future
holds).

Fixes https://github.com/paritytech/polkadot-sdk/issues/1584
Fixes https://github.com/paritytech/polkadot-sdk/issues/2055

---------

Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>

polkadot/runtime/westend/src/lib.rs

@@ -1626,7 +1626,7 @@ mod benches {
 		[pallet_whitelist, Whitelist]
 		[pallet_asset_rate, AssetRate]
 		// XCM
-		[pallet_xcm, XcmPallet]
+		[pallet_xcm, PalletXcmExtrinsiscsBenchmark::<Runtime>]
 		// NOTE: Make sure you point to the individual modules below.
 		[pallet_xcm_benchmarks::fungible, XcmBalances]
 		[pallet_xcm_benchmarks::generic, XcmGeneric]
@@ -2145,6 +2145,7 @@ sp_api::impl_runtime_apis! {
 			use pallet_session_benchmarking::Pallet as SessionBench;
 			use pallet_offences_benchmarking::Pallet as OffencesBench;
 			use pallet_election_provider_support_benchmarking::Pallet as ElectionProviderBench;
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
 			use frame_system_benchmarking::Pallet as SystemBench;
 			use pallet_nomination_pools_benchmarking::Pallet as NominationPoolsBench;
 
@@ -2172,12 +2173,37 @@ sp_api::impl_runtime_apis! {
 			use pallet_session_benchmarking::Pallet as SessionBench;
 			use pallet_offences_benchmarking::Pallet as OffencesBench;
 			use pallet_election_provider_support_benchmarking::Pallet as ElectionProviderBench;
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
 			use frame_system_benchmarking::Pallet as SystemBench;
 			use pallet_nomination_pools_benchmarking::Pallet as NominationPoolsBench;
 
 			impl pallet_session_benchmarking::Config for Runtime {}
 			impl pallet_offences_benchmarking::Config for Runtime {}
 			impl pallet_election_provider_support_benchmarking::Config for Runtime {}
+			impl pallet_xcm::benchmarking::Config for Runtime {
+				fn reachable_dest() -> Option<MultiLocation> {
+					Some(crate::xcm_config::AssetHub::get())
+				}
+
+				fn teleportable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// Relay/native token can be teleported to/from AH.
+					Some((
+						MultiAsset { fun: Fungible(EXISTENTIAL_DEPOSIT), id: Concrete(Here.into()) },
+						crate::xcm_config::AssetHub::get(),
+					))
+				}
+
+				fn reserve_transferable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// Relay can reserve transfer native token to some random parachain.
+					Some((
+						MultiAsset {
+							fun: Fungible(EXISTENTIAL_DEPOSIT),
+							id: Concrete(Here.into())
+						},
+						crate::Junction::Parachain(43211234).into(),
+					))
+				}
+			}
 			impl frame_system_benchmarking::Config for Runtime {}
 			impl pallet_nomination_pools_benchmarking::Config for Runtime {}
 			impl runtime_parachains::disputes::slashing::benchmarking::Config for Runtime {}