PLT-492 Indices for Fx

Add FxIndex and standard constants.
Represents a well-known formalized published FX rate, typically daily.

Extends the principles of Ibor and Overnight rate indices to FX.
The 2 on its own looks a little lonely. Perhaps
```
DaysAdjustment maturity =
    DaysAdjustment.ofBusinessDays(2, baseHoliday.combineWith(counterHoliday));
```