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/xcm/pallet-xcm/Cargo.toml

@@ -13,7 +13,6 @@ scale-info = { version = "2.10.0", default-features = false, features = ["derive
 serde = { version = "1.0.188", optional = true, features = ["derive"] }
 log = { version = "0.4.17", default-features = false }
 
-frame-benchmarking = { path = "../../../substrate/frame/benchmarking", default-features = false, optional = true }
 frame-support = { path = "../../../substrate/frame/support", default-features = false}
 frame-system = { path = "../../../substrate/frame/system", default-features = false}
 sp-core = { path = "../../../substrate/primitives/core", default-features = false}
@@ -25,8 +24,12 @@ xcm = { package = "staging-xcm", path = "..", default-features = false }
 xcm-executor = { package = "staging-xcm-executor", path = "../xcm-executor", default-features = false }
 xcm-builder = { package = "staging-xcm-builder", path = "../xcm-builder", default-features = false }
 
+# marked optional, used in benchmarking
+frame-benchmarking = { path = "../../../substrate/frame/benchmarking", default-features = false, optional = true }
+pallet-balances = { path = "../../../substrate/frame/balances", default-features = false, optional = true }
+
 [dev-dependencies]
-pallet-balances = { path = "../../../substrate/frame/balances" }
+pallet-assets = { path = "../../../substrate/frame/assets" }
 polkadot-runtime-parachains = { path = "../../runtime/parachains" }
 polkadot-parachain-primitives = { path = "../../parachain" }
 
@@ -39,6 +42,7 @@ std = [
 	"frame-support/std",
 	"frame-system/std",
 	"log/std",
+	"pallet-balances/std",
 	"scale-info/std",
 	"serde",
 	"sp-core/std",
@@ -53,6 +57,7 @@ runtime-benchmarks = [
 	"frame-benchmarking/runtime-benchmarks",
 	"frame-support/runtime-benchmarks",
 	"frame-system/runtime-benchmarks",
+	"pallet-assets/runtime-benchmarks",
 	"pallet-balances/runtime-benchmarks",
 	"polkadot-parachain-primitives/runtime-benchmarks",
 	"polkadot-runtime-parachains/runtime-benchmarks",
@@ -63,6 +68,7 @@ runtime-benchmarks = [
 try-runtime = [
 	"frame-support/try-runtime",
 	"frame-system/try-runtime",
+	"pallet-assets/try-runtime",
 	"pallet-balances/try-runtime",
 	"polkadot-runtime-parachains/try-runtime",
 	"sp-runtime/try-runtime",