feat: update currency_code to be optional

[olliemath]

Draft: this allows explicitly setting a quantity (e.g. a rate) as a dimenensionless number by not providing a currency_code

Previously we have insisted every contribution_amount be accompanied by a currency to avoid it being accidentally omitted.

However, this lead to a situation where people started placing non-monetary floats into the text field in order to avoid currency conversion. This in turn caused issues with ids that look like numbers: 1234E5432, 0001000 etc.

examples

// 1.23 USD
{contribution_amount: 123, currency_code: "GBP"}
// 1.23 literal (no unit)
{contribution_amount: 1.23}
// "1.23" (literal string / id)
{contribution_text: "1.23"}

anything else?

this change requires updating the amount field from integer to float - we may want a way to further validate the int status of monetary values

we may want to explicitly version this change as it means we will explicitly prohibit the use of contribution_text for numbers