pallet-xcm: add reserve_withdraw_assets() support

pallet-xcm already offers `reserve_transfer_assets()` call for easy
reserve-based asset transfer, where original assets are reserved in
some reserve location and equivalent assets are minted at destination.

This commit adds `reserve_withdraw_assets()` call for users to be able
to do the operation reverse too: burn equivalent assets and release
original ones from reserve location.

Co-authored-by: Aleksandr Krupenkin <[email protected]>