Consider making constants for currencies

[scr-oath]

I see "USD" in many places… there may be others… we should consider what the strategy is for enum-like constants and use it for this.

I personally like https://github.com/abice/go-enum as a generated enum, which has json/yaml marshaling and the ability to add your own template additions from the structured awareness of the enum metadata.

• See New Adapter: AdUp Tech #4076 (comment)

[bretg]

Interesting idea @scr-oath . We discussed in committee but we're thinking that since USD is the only currency mentioned in the codebase it doesn't make sense to add an enum for any more. And there's concern about the switching back and forth of enums to strings for currencies.

Any followup thoughts?