Refactor the asset-conversion-tx-payment pallet (#14558)

* Code refactoring

* Fix imports

* Typo

* Update frame/asset-conversion/src/types.rs

Co-authored-by: joe petrowski <[email protected]>

* Sync docs

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Co-authored-by: parity-processbot <>
Co-authored-by: joe petrowski <[email protected]>