Make currency user-selectable

1.0 Problem/Motivation
We'd like currency to be user-selectable. We recognize that it's not practical for most folks, because the bank account tied to the payment processor controls what currency is accepted. However, we plan to change this via Twig (or with Webform variants) to match different payment processors. "Pay Later" folks could also select freely.

3.0 Proposed resolution

I also took an initial look. The functionality itself is straightforward. The issue is maintaining the current display of currencies - e.g. "$ USD" and not "United States Dollar". This is done via a custom function, which no "normal" field does. I think in `Fields.src` we want to create a new property "optionCallback" similar to "valueCallback" that allows populating the options via a callback function.

We can use the existing call to `CRM_Core_OptionGroup` which has the added benefit of restricting us to currencies enabled in Civi.

@KarinG this is the issue we'd like to sponsor.