Use MultiLocation for tx fee payment instead of u32

[Chralt98]

What does it do?

The asset id of pallet_asset_tx_payment is replaced by MultiLocation from u32. This is useful to adapt to the newest polkadot-api and polkadot-extension since their default implementation expects a MultiLocation.

What important points should reviewers know?

pallet_asset_tx_payment will fail for a transaction in the standalone mode. However, using the transaction_payment pallet, ZTG can still be used in this case.

The PR was tested against a local network forked using chopsticks. I could execute transactions using USDC from AssetHub as fee payment asset.

It is important to know that in case the Polkadot API team doesn't incorporate the suggested PR from us, we could still fix the issue mentioned below by using an override as implemented here.

UPDATE: It is important to use a polkadot API types version that includes this PR, otherwise we would need the override described above. This is the case for a polkadot api version higher than or equal to 15.5.2.

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues?

polkadot-js/api#5846 (comment)
polkadot-js/api#6074

polkadot-js/api#6081
polkadot-js/api#6083

References

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.19%. Comparing base (ca88b16) to head (7bfcc56).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1403   +/-   ##
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  Coverage   93.19%   93.19%           
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  Files         131      131           
  Lines       29241    29241           
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  Hits        27252    27252           
  Misses       1989     1989           

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