create grant and/or bounty with Payment enum instead of BalanceOf<T> for Currency generic type

[4meta5]

see pallets/drip to define recurring payment metadata

pub enum Payment<Currency, BlockNumber> {
     // dripped with .0 amount sent every .1 blocks until the full amount .2 is paid
     RateUpToFullAmount(Currency, BlockNumber, Currency),
     FullAmountUpFront(Currency),
}