Introduce Payout unit and payin unit enums

[jiyoonie9]

To describe payoutUnitsPerPayinUnit, the spec says something to the effect of "smallest unit of measurement" however we have used dollars and btc to describe say USD/BTC. Using smallest unit of measurement would mean we'd use cents and sats. But that isn't always the "typical" measurements used in a USD/BTC rate.

We should probably remove that wording in the spec and either

1. introduce two new fields (payin/out currency unit)
2. Modify payin/out currency to include unit of measurement used to describe payoutUnitsPerPayinUnit (ie USD_Dollars)