Change the type of currency on IssuedCurrencyAmount from a String to a BigDecimal?

[sappenin]

From @nkramer44:

Maybe this is worth a separate issue, but maybe we could deprecate this in favor of a BigDecimal typed field. I'm assuming there is a way to configure Jackson to deserialize scientific notation correctly, which would make this field much more useful.

I like that in principle because using a number here seems more "correct" -- although the one downside of that is that a developer couldn't easily use a String here in their own code (they would have to translate to a BigDecimal). Is that a problem?

[sappenin]

For context, this thread was inspired by this PR feedback.