Add multiplier to NativeTokenDestination

[geoff-vball]

Context and scope
Some ERC20 contracts have decimals of something other than 18. Currently, sending 1 full token from an ERC20 contract with 24 decimals, will result in 10^6 full tokens being minted on the destination. We should make this configurable.

Discussion and alternatives
We decided that this should be handled on the destination chain only, as the source chain does not need to know the implementation specifics of the destination, and with this implementation, we can unify the interfaces among all the different token bridge contracts.

Open questions