fix: ChargeAssetTxPayment to work with Asset Conversion on Westend's and Kusama's Asset Hubs

[bee344]

Since the addition of the new pallet-asset-conversion to Kusama Asset Hub and Westend Asset Hub, the ChargeAssetTxPayment mutated to accept a MultiLocation as the ID of the asset used to pay the tx fees, which in turn broke the decoding of the blocks in which it was present. What this PR does is fix that by modifying the signed extension for the cases of KAH and WAH, by integrating an alias that modifies the SE to use AssetId or MultiLocation depending on the runtime version. At the same time, as the introduction of the pallet was at different times on KAH and WAH and was not added in Polkadot Asset Hub, it adds type override files for each of them, and leaves the statemint one without change.
It also modifies SignerOptions to accept an object for the SE assetId, so that a MultiLocation can be passed and the SE be used in a straightforward manner.

Block retrieved with substrate-sidecar pre-fix:

Block retrieved with substrate-sidecar after fix:

Closes Issue 5750 and 5734 (probably).

[TarikGul]

I can confirm this fixes the decoding issue we've been seeing with the assetId as a multilocation.

[jacogr]

Seems sane, thank you.

[jacogr]

Some small nigglies identified by CI, see https://github.com/polkadot-js/api/actions/runs/6870177012/job/18703510196?pr=5752#step:5:1294

It basically just needs a default type definition for the added type - generally we probably want to point this to the latest definition, which also means that the higher-version specs don't need the override.

So basically -

1. add a default to point to Option<MultiLocation>
2. keep the override for lower versions (as required)
3. remove the override for current versions (meaning we don't need to keep adding them into the future)

[bee344]

@jacogr I added the default types as you suggested and it solved the issues, but I also had to modify the Signer test to account for the assetId being a MultiLocation

[jacogr]

@bee344 All good, I appreciate your persistence :)

[polkadot-js-bot]

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