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/README.md

@@ -270,7 +270,7 @@ cd <polkadot-sdk-git-repo-dir>
 
 ### Send messages - transfer asset over bridge (ROCs/WNDs)
 
-Do (asset) transfers:
+Do reserve-backed transfers:
 ```
 cd <polkadot-sdk-git-repo-dir>
 
@@ -291,6 +291,20 @@ cd <polkadot-sdk-git-repo-dir>
 	- AssetHubWestend (see `foreignAssets.Issued`, `xcmpQueue.Success`) https://polkadot.js.org/apps/?rpc=ws://127.0.0.1:9010#/explorer
 	- BridgeHubRocococ (see `bridgeWestendMessages.MessagesDelivered`) https://polkadot.js.org/apps/?rpc=ws://127.0.0.1:8943#/explorer
 
+Do reserve withdraw transfers: (when previous is finished)
+```
+cd <polkadot-sdk-git-repo-dir>
+
+# wrappedWNDs from Rococo's Asset Hub to Westend's.
+./cumulus/scripts/bridges_rococo_westend.sh withdraw-reserve-assets-from-asset-hub-rococo-local
+```
+```
+cd <polkadot-sdk-git-repo-dir>
+
+# wrappedROCs from Westend's Asset Hub to Rococo's.
+./cumulus/scripts/bridges_rococo_westend.sh withdraw-reserve-assets-from-asset-hub-westend-local
+```
+
 ### Claim relayer's rewards on BridgeHubRococo and BridgeHubWestend
 
 **Accounts of BridgeHub parachains:**