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 wording here is confusing and at first I thought we were abusing HolidayCalendar to hold something different. Perhaps, "Get the holiday calendar for the index. The rate will be fixed on each non-holiday day" or similar