asset-rate pallet: box asset kind parameter (#1545)

The `AssetKind` type parameter of a dispatchable, defined by the user,
might be large — like `xcm::MultiLocation`. To prevent inflating the
size of the `Call` type, we `Box` it.

This changes required for
https://github.com/paritytech/polkadot-sdk/pull/1333



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Co-authored-by: Oliver Tale-Yazdi <[email protected]>