Add assets in pool with native to query_acceptable_payment_assets's return (#5599)

`XcmPaymentApi::query_acceptable_payment_assets` is an API that returns
the assets that can be used to pay fees on the runtime where it's
called.
For relays and most system chains this was configured only as the native
asset: ROC and WND.
However, the asset hubs have the asset conversion pallet, which allows
fees to be paid in any asset in a pool with the native one.
This PR adds the list of assets in a pool with the native one to the
return value of this API for the asset hubs.

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Co-authored-by: Branislav Kontur <[email protected]>