Be able to specify the fee aside the amount to be transferred in Xtokens

[girazoki]

This is a usecase we encountered in Moonbeam, where we need to specify the fee (the amount to be provided in BuyExecution) as a separate argument to the amount being transferred, with the goal of better controlling the amount received in the destination side. We are in the process of adding this to Xtokens in our own fork, would you be interested on having such feature too? I could do a PR with my changes if you are interested

[girazoki]

Just FYI, I am making this changes as an additional functionality (by adding two new extrinsics), I am not changing any of the existing functionality in Xtokens

[xlc]

Happy to accept PR as long as it is transaction version compatible.

[girazoki]

This is a first draft, let me know if this is something you would like to add #658