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.
This would be clearer if you used the ``calculateMaturityFromFixing`` method to do the checking? So
```
while (calculateMaturityFromFixing(fixingDate).isAfter(effectiveBusinessDay)) {
  fixingDate = fixingDate.minusDays(1);
}
```
(Assuming it doesn't vastly increase the calculation complexity)