XCM WeightTrader: Swap Fee Asset for Native Asset (#1845)

Implements an XCM executor `WeightTrader`, facilitating fee payments in
any asset that can be exchanged for a native asset.

A few constraints need to be observed:
- `buy_weight` and `refund` operations must be atomic, as another weight
trader implementation might be attempted in case of failure.
- swap credit must be utilized since there isn’t an account to which an
asset of some class can be deposited with a guarantee to meet the
existential deposit requirement. Also, operating with credits enhances
the efficiency of the weight trader -
https://github.com/paritytech/polkadot-sdk/pull/1677

related PRs:
- (depends) https://github.com/paritytech/polkadot-sdk/pull/2031
- (depends) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1847
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

// DONE: impl `OnUnbalanced` for a `fungible/s` credit
// DONE: make the trader free from a concept of a native currency and
drop few fallible conversions. related issue -
https://github.com/paritytech/polkadot-sdk/issues/1842
// DONE: tests

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

Cargo.lock

@@ -1066,6 +1066,7 @@ dependencies = [
  "frame-support",
  "frame-system",
  "hex-literal",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-collator-selection",
@@ -1973,6 +1974,7 @@ dependencies = [
  "frame-support",
  "hex",
  "hex-literal",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-bridge-messages",
@@ -2165,6 +2167,7 @@ dependencies = [
  "cumulus-pallet-xcmp-queue",
  "emulated-integration-tests-common",
  "frame-support",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-bridge-messages",
@@ -4095,6 +4098,7 @@ dependencies = [
  "cumulus-primitives-core",
  "frame-support",
  "log",
+ "pallet-asset-conversion",
  "pallet-xcm-benchmarks",
  "parity-scale-codec",
  "polkadot-runtime-common",