Add an adapter for configuring AssetExchanger (#5130)

Added a new adapter to xcm-builder, the `SingleAssetExchangeAdapter`.
This adapter makes it easy to use `pallet-asset-conversion` for
configuring the `AssetExchanger` XCM config item.

I also took the liberty of adding a new function to the `AssetExchange`
trait, with the following signature:

```rust
fn quote_exchange_price(give: &Assets, want: &Assets, maximal: bool) -> Option<Assets>;
```

The signature is meant to be fairly symmetric to that of
`exchange_asset`.
The way they interact can be seen in the doc comment for it in the
`AssetExchange` trait.

This is a breaking change but is needed for
https://github.com/paritytech/polkadot-sdk/pull/5131.
Another idea is to create a new trait for this but that would require
setting it in the XCM config which is also breaking.

Old PR: https://github.com/paritytech/polkadot-sdk/pull/4375.

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>