Define MinXcmFee to allow paying fee in non-reserve asset

[ebma]

Context

It's currently not possible to transfer tokens with the xtokens pallet while paying the transfer fee with the non-reserve asset on Pendulum. For example, the xtokens::transferMultiCurrencies() call will fail with MinXcmFeeNotDefined when transferring an asset back to Moonbeam and choosing PEN as the fee asset, see this extrinsic.

This is useful

TODO

Define this type on the xtokens config so that non-reserve assets can be used for paying the XCM fee.