Refactor: simplify XCM weight trading

[mrshiposha]

Description

This is a draft of an XCM weight trading refactoring that simplifies its API and makes it re-usable in the XCM payment API for converting weight to a fee in a selected fungible currency.

A common implementation for XCM payment API's query_weight_to_asset_fee is added to pallet-xcm.

The relevant tests in the xcm-builder and asset-hub-westend runtime are updated.

CC @acatangiu @franciscoaguirre

Integration

The PR changes the existing WeightTraders and adds a new entry to the pallet-xcm's config.
It affects the Runtime builders.

Review Notes

This is a draft, so some things aren't polished out. It is intentional because it seems reasonable to show the general idea first and collect feedback before finalizing everything (like changing in-code documentation, addressing TODOs and FIXMEs such as renaming relevant tests, adding logs in some places, etc).

The relevant changes are in the xcm-executor, xcm-builder, cumulus-primitives-utility, pallet-xcm, and asset-hub-westend runtime.
The rest was made only to build the Westend Relay and Westend Asset Hub for basic local testing.

WeightTrader related

1. The xcm-executor's WeightTrader trait has been modified to deal only with fungibles since no obvious mechanism for using NFTs as XCM fees exists:

   • It is unclear how NFTs can be used for delivery fees
   • The XCM payment API doesn't have methods where NFTs are considered fee assets
     Apparently, no one uses NFTs for XCM execution fees, so it seems reasonable to remove them from the general weight trading API. If a kind of "XCM execution ticket" is needed in the future, corresponding logic could be added separately based on known use cases.

2. The WeightTrader trait now describes a stateless mechanism. The traders' common state is now moved to the xcm-executor itself, represented by a FeePayment struct containing the desired fee asset ID, the actually used fee asset ID (which could be a different one in case of a swap), and the fee paid amount. This allows for avoiding re-implementing the state-tracking logic in all the weight traders, allowing for focusing on the actual weight fee computation in them, making their code clearer.
   The corresponding logic for buying and refunding the weight has been altered in the xcm-executor.

3. The xcm-executor already has the AssetExchanger capable of swapping the assets. The Weight Trader might request an asset swap if the desired fee asset ID can't (or shouldn't) be used directly as a weight fee. The Trader either tells the executor to use the provided desired fee asset ID and returns the corresponding required amount of it via WeightFee::Desired or it tells how much of the desired asset must be swapped and to what it should be swapped via WeightFee::Swap.
   This way, a weight trader's code that allows an asset swap can be kept simple, only quoting the exchange price without changing any state and providing all the relevant info about the weight fee.

4. The WeightTrader trait now has a default implementation for computing the refund weight amount. Most implementations can use it. A special refund weight computation is needed when there is some constraint on the provided payment to be honored (e.g., ensuring that at least the asset's minimum balance will be paid for the weight).

[acatangiu]

I like it high-level.

The WeightTrader trait now describes a stateless mechanism. The traders' common state is now moved to the xcm-executor itself, represented by a FeePayment struct containing the desired fee asset ID, the actually used fee asset ID (which could be a different one in case of a swap), and the fee paid amount. This allows for avoiding re-implementing the state-tracking logic in all the weight traders, allowing for focusing on the actual weight fee computation in them, making their code clearer.
The corresponding logic for buying and refunding the weight has been altered in the xcm-executor.

This part needs more review - this is really the big change proposed here, with new logic proposed that we need to carefully audit..