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>

cumulus/parachains/runtimes/bridge-hubs/bridge-hub-westend/src/lib.rs

@@ -544,7 +544,7 @@ mod benches {
 		[pallet_collator_selection, CollatorSelection]
 		[cumulus_pallet_xcmp_queue, XcmpQueue]
 		// XCM
-		[pallet_xcm, PolkadotXcm]
+		[pallet_xcm, PalletXcmExtrinsiscsBenchmark::<Runtime>]
 		// NOTE: Make sure you point to the individual modules below.
 		[pallet_xcm_benchmarks::fungible, XcmBalances]
 		[pallet_xcm_benchmarks::generic, XcmGeneric]
@@ -772,6 +772,7 @@ impl_runtime_apis! {
 			use frame_support::traits::StorageInfoTrait;
 			use frame_system_benchmarking::Pallet as SystemBench;
 			use cumulus_pallet_session_benchmarking::Pallet as SessionBench;
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
 
 			// This is defined once again in dispatch_benchmark, because list_benchmarks!
 			// and add_benchmarks! are macros exported by define_benchmarks! macros and those types
@@ -813,6 +814,29 @@ impl_runtime_apis! {
 			use cumulus_pallet_session_benchmarking::Pallet as SessionBench;
 			impl cumulus_pallet_session_benchmarking::Config for Runtime {}
 
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
+			impl pallet_xcm::benchmarking::Config for Runtime {
+				fn reachable_dest() -> Option<MultiLocation> {
+					Some(Parent.into())
+				}
+
+				fn teleportable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// Relay/native token can be teleported between BH and Relay.
+					Some((
+						MultiAsset {
+							fun: Fungible(EXISTENTIAL_DEPOSIT),
+							id: Concrete(Parent.into())
+						},
+						Parent.into(),
+					))
+				}
+
+				fn reserve_transferable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// Reserve transfers are disabled on BH.
+					None
+				}
+			}
+
 			use xcm::latest::prelude::*;
 			use xcm_config::WestendLocation;