[REQS-712] Light-weight holiday source proposal

This is pretty bare-bone at this point. I'm just testing the waters, see if that's the right way to go.
Not sure if we need to support this call - I'd certainly prefer not to expose the concept of a Currency holiday to the client if possible. Maybe just throw here for now and we'll see how it goes with the test portfolios. I think the vast majority of pricing code uses region ids to reference holidays so we should try to use this mechanism instead.